Ca­pac­ity dilemma

With more trains us­ing an in­creas­ingly in-de­mand net­work, how can op­er­a­tors and plan­ners keep the trains mov­ing? PHILIP HAIGH re­ports from a con­fer­ence dis­cussing the prob­lem

Rail (UK) - - Con­tents -

With more trains us­ing an in­creas­ingly in-de­mand net­work, how can op­er­a­tors and plan­ners keep the trains mov­ing?

Sub­ur­ban traf­fic on the GWR had al­ways been leisurely by the stan­dards of other lines. So wrote Alan Jack­son in his book Lon­don’s Ter­mini in 1969. “Un­couth ex­pres­sions such as ‘rush hours’ were rarely if ever heard on Great Western tongues,” he said.

He records that in 1903 there were only eight sub­ur­ban trains ar­riv­ing at Padding­ton be­tween 0500 and 1000. Over at Liver­pool Street, there were 136.

There are more trains to­day at Brunel’s ter­mi­nus, but even now some of those ar­rivals com­prise only three-car units. Con­trast that with the 12-car trains ar­riv­ing at many Lon­don sta­tions, dis­gorg­ing their masses on­wards to work.

But Padding­ton is chang­ing. Last year elec­tric oper­a­tions started with four-car Class 387s run­ning in pairs. From 244 seats on a three-car diesel mul­ti­ple unit to 888 on an eight-car elec­tric is quite a change.

Fur­ther change for Padding­ton’s com­muters is com­ing with Cross­rail’s nine-car Class 345s, that will use a new un­der­ground sta­tion hav­ing left the Great Western re­lief lines near Royal Oak for new tracks. Cross­rail will pro­vide di­rect links from Thames Val­ley sta­tions east of Read­ing to Lon­don’s West End and City area.

The con­tin­u­ing roll­out of Hi­tachi Class 800s to Padding­ton’s long-dis­tance ser­vices is lead­ing to ser­vice changes on a scale not

wit­nessed since 1976’s in­tro­duc­tion of High Speed Trains. Main line ser­vice fre­quen­cies change from De­cem­ber 2017’s 12 trains per hour (tph) to 15tph in De­cem­ber 2018, with two more trains ev­ery hour for Bris­tol Tem­ple Meads and an ex­tra hourly de­par­ture that al­ter­nates be­tween Chel­tenham Spa and the West Coun­try. The Re­lief Lines in­crease from 8tph in De­cem­ber 2017 to 12tph in De­cem­ber 2019, with Cross­rail adding two hourly trains to Heathrow Ter­mi­nal 5 and two for Read­ing. Three hourly freight paths re­main.

Net­work Rail ex­pects 0800-0859 de­mand to rise from around 4,000 pas­sen­gers on the Re­lief Lines in 2016 to 16,000 in 2043, while main line de­mand will rise from 8,000 to 17,000 over the same pe­riod (see graph 1). Lines will be full by 2020 in terms of train fre­quency.

That’s why NR is now look­ing to­wards re­mod­elling the sta­tion’s ap­proaches. Bri­tish Rail and Rail­track last did the job in the early 1990s. They opted for a lay­out that pri­ori­tised speed, with turnouts at Lad­broke Grove Junc­tion fit for 50mph as trains switched tracks be­fore ap­proach­ing on bi-di­rec­tional lines.

NR Lead Strate­gic Plan­ner Ben Sturgess re­vealed the prob­lems this causes to­day to a con­fer­ence or­gan­ised by the In­sti­tu­tion of Me­chan­i­cal En­gi­neers on Novem­ber 14. Padding­ton’s throat es­sen­tially stretches 1¾ miles out to Lad­broke Grove, and trains cross­ing (for ex­am­ple) from a high-num­bered plat­form at Padding­ton to the Down Main can block the throat for some time. The prob­lem Sturgess out­lined neatly sum­marises the chal­lenge a busier rail­way brings - there are trade-offs be­tween speed and ca­pac­ity, be­tween ca­pac­ity and per­for­mance, and be­tween per­for­mance and speed.

If Padding­ton is to be full in train terms by 2020, then what of mak­ing trains longer as

While plan­ners care­fully con­struct timeta­bles to re­move con­flicts be­tween trains, the more com­plex a timetable is the less re­silient it will be.

Brat­ton ad­vised that the best way to min­imise in­ter­ac­tion was to con­cen­trate on in­creas­ing the cer­tainty of trains be­ing in the right place at key points on the net­work. For this rea­son, he re­jected early run­ning.

the usual op­tion for in­creas­ing ca­pac­ity? Tricky. Padding­ton tracks curve sharply at their coun­try end, giv­ing a limit of around 300 me­tres for trains. Tracks could be ex­tended to­wards the con­course, but this cuts cir­cu­lat­ing space and would lead to crowd­ing, Sturgess ex­plained.

So re­mod­elling beckons. This could be com­pli­cated be­cause NR sug­gests build­ing a fly­ing junc­tion at Lad­broke Grove. Back in 2015, it priced the work at be­tween £ 75 mil­lion and £175m, and reck­oned its value for money to be ‘high’. At the time, it said that an en­hanced junc­tion would per­mit bet­ter use of net­work ca­pa­bil­ity and sup­port bet­ter per­for­mance. Build­ing a dive-un­der or fly­over repli­cates the ma­jor work re­cently done by NR at Read­ing, that has im­proved per­for­mance by re­duc­ing con­flict­ing moves.

Sturgess sug­gested that im­prove­ments at Lad­broke Grove would need to be matched by sim­i­lar grade sep­a­ra­tion at Did­cot East (where trains to and from Ox­ford di­verge from the Great Western Main Line). In 2015, NR rated this plan as poor value for money, but said it couldn’t iden­tify all the ben­e­fits it might bring (al­though it would bring bet­ter punc­tu­al­ity).

The twin-track line from Did­cot to Swin­don would also need im­prov­ing, he said. This could be done by ex­tend­ing the four-mile loops be­tween Chal­low and Wan­tage Road to al­low faster trains to over­take slower ones.

Punc­tu­al­ity de­clines in Padding­ton’s morn­ing and evening peaks, re­cov­er­ing be­tween them. The drop is more marked for ‘right-time’ than for PPM (Pub­lic Per­for­mance Mea­sure) tar­gets of within ten min­utes for long-dis­tance trains and five min­utes for sub­ur­ban ser­vices.

This sug­gests a link be­tween ca­pac­ity and per­for­mance. But when Sturgess showed the IMechE au­di­ence a graph of punc­tu­al­ity against trains per hour for Lon­don morn­ing peak ar­rivals, there was a spread that re­vealed no cor­re­la­tion (see graph 2). Plot­ting ser­vice fre­quency against speed showed that as speeds rise, the num­ber of trains per hour falls. Sturgess ex­plained that as speed in­creases, so must sig­nal spac­ing, which in­creases head­ways (the dis­tance be­tween trains on the same track) and re­duces ca­pac­ity.

He went on to ex­plain that faster trains took less time to clear a sig­nalling block sec­tion, but that as speeds in­creased so sig­nal spac­ing also had to, but at the square of the speed. The re­sult is a graph that shows for four-as­pect sig­nalling the low­est head­ways oc­cur­ring at around 30mph (see graph 3). As speeds in­crease, so does the time taken to re­cover from per­tur­ba­tion, such as a sig­nal stop. Sturgess said that a stop from 60mph costed 40 sec­onds, but one from 125mph costed over two min­utes (see graph 4).

While plan­ners care­fully con­struct timeta­bles to re­move con­flicts be­tween trains, the more com­plex a timetable is the less re­silient it will be. Trains graphs typ­i­cally plot ser­vices as slop­ing lines on axes of time and dis­tance. Trains run­ning at the same speed will have the same slope on the graph. Plan­ners can plot these as close to­gether as sig­nalling al­lows.

A slower train will have a shal­lower slope and take up more space on the graph, and so more ca­pac­ity on the track (see graph 5). Sturgess showed that per­for­mance falls as the dif­fer­ence in speed be­tween trains in­creases. This is tra­di­tion­ally why busy lines had fast lines and slow lines. As the slow­est freight trains dis­ap­peared, so BR re­moved ex­tra lines. To­day, the dif­fer­ence is re­turn­ing with

By find­ing the prob­lems that cause de­lays that are oth­er­wise in­vis­i­ble to staff or pas­sen­gers, an op­er­a­tor could in­crease ca­pac­ity or im­prove per­for­mance.

the dif­fer­ence be­tween stop­ping trains and ex­presses.

That is why trains are of­ten ‘flighted’ from a sta­tion. The fastest leaves first and the oth­ers fol­low in or­der of de­creas­ing speed. There is then a gap in de­par­tures un­til the next flight leaves. Out on the track, the trains within the flight are grad­u­ally draw­ing fur­ther apart. The slow­est must have ar­rived some­where clear of the line be­fore the fastest of the fol­low­ing flight reaches it.

In the days be­fore com­put­ers, a train plan­ner would have done all this on pa­per with a sharp pen­cil, its line rep­re­sent­ing the train pre­cisely at any point in dis­tance and time. But trains do not al­ways run pre­cisely - over sev­eral days, weeks or months a ser­vice might run early or late. Mostly, it will be on time, but its per­for­mance will vary (see graph 6).

That plan­ner’s sharp pen­cil may be bet­ter re­placed by a thick felt-tip pen, with its wide line giv­ing a bet­ter im­pres­sion of how a train re­ally runs. If, on a five-minute fre­quency, one train runs two min­utes late and the fol­low­ing one three min­utes early, then both will be try­ing to be at the same point. Of course, sig­nalling will slow or stop the fol­low­ing early train, but that ac­tion in­tro­duces per­tur­ba­tion that can rip­ple across a timetable.

Map­ping how trains per­form was the sub­ject tack­led by MTR Eu­ro­pean Oper­a­tions Di­rec­tor Oliver Brat­ton. He recog­nised that trains per­form within a range of ‘on-time’, usu­ally on-time, some­times late, and oc­ca­sion­ally early. How a train will ac­tu­ally per­form de­pends on prob­a­bil­ity, and this makes it hard to de­fine point-to-point tim­ings. If tim­ings can­not be pre­cisely de­fined, it makes cre­at­ing timetable mod­els much harder.

MTR mapped per­for­mance us­ing train graphs, prob­a­bil­ity and pas­sen­ger load­ings (vari­able pas­sen­ger num­bers at sta­tions make it harder to use stan­dard dwell times in mod­els, even for a sin­gle sta­tion). This helped vi­su­alise per­for­mance and make the most of space be­tween trains, but even this didn’t ac­count for how one train might in­ter­act with an­other.

One train might de­lay an­other, or a third might de­lay one of them. If both are late, there might be no in­ter­ac­tion. Even mod­el­ling two trains on a sim­ple sec­tion of track with dif­fer­ent run­ning times can bring a large range of re­sults in terms of de­lay if one train is late, par­tic­u­larly if the sec­ond train is al­lowed to go first as a re­sult of the first be­ing de­layed.

It led Brat­ton to ask whether rail­way com­pa­nies un­der­stand how run­ning one train af­fects an­other, and whether they un­der­stand which per­for­mance is be­ing reg­u­lated for. Add more trains, and mod­el­ling and vi­su­al­i­sa­tion be­comes even more com­pli­cated. Given this, he ad­vised that the best way to min­imise in­ter­ac­tion was to con­cen­trate on in­creas­ing the cer­tainty of trains be­ing in the right place at key points on the net­work. For this rea­son, he re­jected early run­ning.

To dis­cover the prob­a­bil­ity of a train be­ing where it is sup­posed to be needs good data. This has been the sub­ject of much of Paul Nay­lor’s work at CPC Project Ser­vices.

Nay­lor has been work­ing with Lon­don Un­der­ground’s Ju­bilee Line. The line is twin-track, 22 miles long, and has 25 sta­tions be­tween Stan­more and Strat­ford. Trains op­er­ate un­der TBTC (trans­mis­sion-based train con­trol) with ATO (au­to­matic train op­er­a­tion), rather than un­der driv­ers’ man­ual con­trol. Nay­lor said the sys­tem col­lected 5GB of train­run­ning data ev­ery day.

He took a typ­i­cal day ( Jan­uary 16 2017), with a 30tph ser­vice run­ning un­der 120-sec­ond head­ways. The day had three ser­vice de­lays that to­talled 11 min­utes. Us­ing a two-minute thresh­old, de­lays over the day reached six hours. Plot­ting trains be­ing de­layed and re­cov­er­ing time showed the most gained time at Stan­more, with de­lays con­fined to a suc­ces­sion of trains head­ing west from Baker Street to Stan­more around 0800, just af­ter the first ser­vice de­lay.

In­creas­ing the vis­i­bil­ity of de­lays by re­duc­ing the thresh­old to 60 sec­onds showed most trains pass­ing Ca­nary Wharf west­bound be­tween 1700 and 1830 be­ing de­layed. A re­duc­tion to ten sec­onds left Nay­lor’s chart lit­tered with red and blue dots, rep­re­sent­ing trains de­layed or early. It showed that al­most ev­ery train was late ap­proach­ing West Hamp­stead when head­ing west. In the other di­rec­tion, most trains were early at Willes­den Green but late at the next sta­tion, Kil­burn.

These ex­am­ples come from ap­ply­ing the ten-sec­ond de­lay fil­ter run­ning times be­tween sta­tions, and show that the timetable doesn’t match real run­ning. When it’s ap­plied to dwell times at sta­tions, it shows that peak dwell times are not suf­fi­cient to ac­count for the num­ber of pas­sen­gers in the cen­tral sec­tion

be­tween Baker Street and Ca­nary Wharf. With skilled anal­y­sis, the charts could be used to iden­tify where timeta­bles gave in­suf­fi­cient time for a train to en­ter a re­vers­ing sid­ing, or the ef­fect of in­ter­lock­ing re­lease times, or of doors open­ing too slowly. Nay­lor cau­tioned that an­a­lysts needed good sys­tem and op­er­at­ing knowl­edge to ef­fec­tively drill into the de­tail of de­lays, but with this they could find prob­lems at junc­tions and sid­ings, power faults or poorly per­form­ing trains. By find­ing the prob­lems that cause de­lays that are oth­er­wise in­vis­i­ble to staff or pas­sen­gers, an op­er­a­tor could in­crease ca­pac­ity or im­prove per­for­mance.

Com­pared with many NR routes, or even some of LU’s lines such as its sub-sur­face net­work of Met­ro­pol­i­tan, Ham­mer­smith and City and Cir­cle Lines, the Ju­bilee is a sim­ple net­work. Yet it pro­duces a large amount of data ev­ery day. Scale this up­wards to more com­pli­cated lines and the data will in­crease mas­sively. Even so, Nay­lor clearly showed that ex­am­in­ing it is the key to solv­ing many prob­lems.

Net­work Rail Ca­pac­ity Plan­ning Di­rec­tor Chris Row­ley pro­vided the con­fer­ence with a dif­fer­ent view of per­for­mance. He showed what the com­pany had found from fly­ing a drone above Wind­mill Bridge Junc­tion in South Lon­don one af­ter­noon, when there was no dis­rup­tion. It re­vealed that be­tween 1630 and 1815, 39 trains were forced to stop at the junc­tion.

Wind­mill Bridge is com­plex. It ef­fec­tively in­cludes Sel­hurst, Cot­tage, Nor­wood Fork and Gloucester Road Junc­tions. It per­mits trains head­ing north from East Croy­don to di­verge for Lon­don Bridge or Vic­to­ria. It of­fers the same op­tion for trains from West Croy­don. It Great Western Rail­way 387133 waits at Padding­ton on March 3 2017. JACK BOSKETT/ RAIL. also switches the lay­out of tracks from be­ing paired by speed to be­ing paired by di­rec­tion.

There is con­sid­er­able po­ten­tial for de­lays to spread from a junc­tion this com­pli­cated. Row­ley used the phrase “spread like wild­fire”, and ex­plained that it was very dif­fi­cult to add time to ser­vices or cre­ate fire­breaks.

Wind­mill Bridge goes some way to ex­plain­ing why per­for­mance for the lead op­er­a­tors on NR’s South East route has been con­sis­tently be­tween two and ten per­cent­age points below sim­i­lar sub­ur­ban op­er­a­tors on Wes­sex and Anglia, ac­cord­ing to Row­ley. Plenty of peo­ple have tried to im­prove per­for­mance, and there have been sev­eral emer­gency re­views. “There’s clearly a fun­da­men­tal prob­lem down there,” said Row­ley, be­fore re­veal­ing what he thought it was.

He showed his IMechE au­di­ence a graph of the top 19 busiest Net­work Rail junc­tions (see graph 7). Twelve sit on Sus­sex and the wider South East route, with 27 trains cross­ing per hour at the busiest - Ewer Street be­tween Water­loo East and Lon­don Bridge.

NR has plans to re­model Wind­mill Bridge Junc­tion. Re­vealed in 2015, they would add two plat­forms to East Croy­don, and re­model its lay­out to pro­vide eight rather than six through tracks, which then feed into a re­vised Wind­mill Bridge that fea­tures new bridges. A note at­tached to the plan stresses that it is only in­dica­tive and will be sub­ject to de­tailed de­sign, fund­ing, op­er­a­tional anal­y­sis and

If links be­tween timeta­bles are be­com­ing tighter, so the rail­way must con­sider links be­tween projects, par­tic­u­larly if Net­work Rail is to con­tinue de­liv­er­ing up­grade schemes as well as main­tain­ing and re­new­ing the rail­way it al­ready has.

plan­ning pow­ers.

The lines through Wind­mill Bridge Junc­tion will soon be linked to the East Coast Main Line via Canal Tun­nels near St Pan­cras. Ac­cord­ing to one se­nior rail­way­man, ECML per­for­mance can be “car­nage” on a bad day. Cur­rent Mov­ing An­nual Av­er­age per­for­mance for Vir­gin Trains East Coast is 84.7%, with only Hull Trains (an­other ECML user, 79.9%) and Govia Thames­link Rail­way (GTR, 79.5%) below it. GTR in­cludes Thames­link, South­ern and Gatwick Ex­press ser­vices over Wind­mill Bridge and Great North­ern trains on the ECML.

How de­lays might be trans­mit­ted from one side of Canal Tun­nels to the other re­mains to be seen. Thames­link trains run­ning through the core un­der cen­tral Lon­don will be run­ning in au­to­matic mode. Leav­ing the core, they will fan out­wards to sev­eral des­ti­na­tions much as wa­ter leaves a hose spray. The trick to punc­tu­al­ity is fun­nelling the spray into the hose at the other end of the core.

Cross­rail has the same prob­lem. In the cen­tral sec­tion, its Class 345s will run au­to­mat­i­cally un­der CBTC (com­mu­ni­ca­tions-based train con­trol), but Oliver Brat­ton noted that this sys­tem does not look beyond its cen­tral bound­aries to see how trains are ap­proach­ing. It can’t ad­just their run­ning as they ap­proach, to en­sure they are in the cor­rect or­der and on time. Thus Brat­ton ad­mit­ted that no one knew whether CBTC would trans­mit de­lays from ei­ther side of the cen­tral sec­tion or ab­sorb them. Thames­link faces the same sit­u­a­tion.

Brat­ton sug­gested that not all prob­lems can be solved by com­put­ers. Mod­els of plan­e­tary mo­tion pro­duce re­sults that as­tronomers know are wrong but good enough. Re­sults from rail mod­els were sim­i­larly good enough, but on a busier net­work their flaws be­gin to be ex­posed. Cur­rent rail com­puter mod­els take a long time to build, and (as Row­ley noted) their re­sults can of­ten come too late to change any­thing. He ar­gued that ex­pert judge­ment re­mained the most im­por­tant area, but that this needed an ac­cu­rate de­scrip­tion of the changes be­ing pro­posed.

The long timescales in­volved make it dif­fi­cult to try dif­fer­ent things to find what works best. In­creas­ing links be­tween dif­fer­ent lines and routes makes mod­el­ling one in iso­la­tion much harder.

This is why Net­work Rail is look­ing to cre­ate a new model us­ing the huge amount of train run­ning data it col­lects. NR Sys­tem Op­er­a­tor Man­ag­ing Di­rec­tor Jo Kaye ad­mits she doesn’t know how this will work out, but the com­pany is talk­ing to aca­demics to dis­cover what might be done.

The idea of a model on which large or small changes can be tested across the net­work is pow­er­ful. It qual­i­fies as an­other ‘holy grail’ of train plan­ning. LU’s Ju­bilee Line ex­pe­ri­ence shows what can be done with data, but NR’s am­bi­tion for its model is vast - not just in the ge­og­ra­phy of its net­work, but also in the daily and hourly vari­a­tions in train run­ning caused by events in­side and out­side the rail­way’s con­trol. Tweak­ing trains around Brighton could rip­ple to­ward Ed­in­burgh via Thames­link; Birm­ing­ham could find it­self linked to Chelms­ford through Cross­Coun­try ser­vices to Read­ing, Cross­rail through Lon­don and onto the Great East­ern Main Line.

At the heart of NR’s prob­lems is that a busier net­work brings stiffer links be­tween ar­eas where once empty space on timetable graphs made links more elas­tic. It’s akin to op­er­at­ing a com­plex me­chan­i­cal ma­chine, full of levers and gears. You need fine tol­er­ances be­tween parts for the ma­chine to run smoothly, but the vari­ables in train per­for­mance mean that your tol­er­ances are wide and sloppy and so the ma­chine jams.

If links be­tween timeta­bles are be­com­ing tighter, so the rail­way must con­sider links be­tween projects, par­tic­u­larly if Net­work Rail is to con­tinue de­liv­er­ing up­grade schemes as well as main­tain­ing and re­new­ing the rail­way it al­ready has.

GWR Man­ag­ing Di­rec­tor Mark Hop­wood told the IMechE con­fer­ence that projects af­fect­ing his trains were de­vel­oped in iso­la­tion from oth­ers. Ear­lier de­liv­ery of the dou­ble-track up­grade from Bices­ter to Ox­ford for trains from Maryle­bone could have helped pas­sen­gers dis­rupted by work be­tween Padding­ton, Read­ing and Did­cot, he ar­gued, even though he rue­fully added that the Bices­ter project had caused GWR some com­mer­cial pain.

Hop­wood ar­gued that adding trains did not al­ways mean a re­duc­tion in per­for­mance. The West Coast Main Line is run­ning more re­li­ably now than be­fore Vir­gin in­tro­duced Class 390s, while the North Lon­don Line has bet­ter per­for­mance and more trains. Both lines were sub­ject to ex­ten­sive up­grade work. The chal­lenge to­day, Hop­wood reck­oned, was that so many up­grades were tak­ing place at the same time

Al­though the Me­chan­i­cals’ con­fer­ence car­ried the ti­tle Per­for­mance or Ca­pac­ity: Ei­ther

or Both? it’s clear that pas­sen­gers and freight for­warders want both. Rail com­pa­nies must plan to de­liver both, but must be pre­pared to ex­plain where com­pro­mises lie.

This was a point NR’s Chris Row­ley made strongly. He said the rail­way in­dus­try was not “done to”. It pro­poses changes and must be able to tell fun­ders, par­tic­u­larly gov­ern­ments, where trade-offs lie.

Money can ex­plain some of these com­pro­mises. Adding ca­pac­ity al­lows rail­way com­pa­nies to sell more of their prod­uct - whether that’s seats, con­tainer space or track paths.

Pas­sen­ger com­pa­nies now have to hand money back to pas­sen­gers when trains are late. The De­part­ment for Trans­port’s ‘De­lay Re­pay’ scheme en­tails a full re­fund for a train an hour or more late, and half your money back for de­lays over 30 min­utes. Re­cent fran­chise awards have re­duced the thresh­old to 15 min­utes. This should cre­ate a strong com­mer­cial link be­tween per­for­mance and ca­pac­ity.

Rail com­pa­nies should de­sign both as­pects into their timeta­bles from the start. And as IMechE Rail­way Divi­sion Chair­man Richard McClean said at the end of Novem­ber 14’s con­fer­ence, they must then con­cen­trate on ba­sic oper­a­tions, main­te­nance and re­newals, and de­liver their prod­uct lo­cally with proper grass-roots un­der­stand­ing.


Great Western Rail­way 43091 passes Royal Oak on its ap­proach to Lon­don Padding­ton on Fe­bru­ary 2 2017. More and more trains are set to use the busy West Lon­don ter­mi­nus.


A busy plat­form at Baker Street on Lon­don Un­der­ground’s Ju­bilee Line on Fe­bru­ary 12 2014. Data col­lected from trains’ per­for­mance on the line has helped re­search into punc­tu­al­ity.

Source: MTR.

Trains ar­riv­ing at a fixed point can vary in punc­tu­al­ity ev­ery day.

Source: Net­work Rail.

Most of Net­work Rail’s busiest junc­tions (in terms of trains per hour, ex­cept at ter­mini) are on its South East Route.

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