The below ERTMS cab display is from a Down train formed of two Class 158 units approaching Machynlleth.

It is a duplicate display provided for people on a technical visit to the ERTMS system.

The quality of cab display picture is not very good, but it was chosen because it illustrates the planning area of the CAB ERTMS.

The speedometer part of the display shows the speed in kilometres per hour at 87kph (54mph). Part of the hook around the speedo needle is in yellow to show a speed reduction is needed down to 25kph (the grey area). This is the permitted speed for the points and station area at Machynlleth. The yellow square with 570 below it indicates there is 570 metres until the commencement of the lower speed.

The area to the right of the picture is the planning area. This area shows the speed profile ahead of the train. The first number from the bottom is 500m, then 1,000m, 2,000m, and at the top 4,000m, so the display is non-linear.

This is to give the driver the detail needed for the line immediately ahead, but also to give a medium-distance view of the line ahead within a sensible size display. The light grey wide bars represent the permitted speeds, so the reduction to 25kph 570 metres ahead shown on the speedo is also shown as the step in the light grey area just above the 500m line.

The 25kph limit through the station then steps back up to linespeed just above the 1,000m line. The thinner grey bar shows the extent of the movement authority (how far the signals are cleared for) that the train has, which in this case is into the platform at Machynlleth.

The 4,000m look ahead provided by the display is roughly equivalent to the distance from sighting a double yellow to the red signal on a 100mph 4-aspect railway. It is much better than the warning given to a driver on lower-speed railways and in areas of 3-aspect signalling. Where CDAS is provided in conjunction with a Traffic Management System the look ahead has the potential to be vastly better.

For example, for a freight train running from March to Ely (a distance of 14 miles):

On conventional signalling:

Accelerate from the 40mph restriction through March station and try to reach the 60mph line speed. On approach to Ely North Junction see restrictive aspects double yellow then single yellow, so reduce speed to around 20mph. Accelerate back up to 40mph for the run into Ely station and the junctions there.

With CDAS:

On leaving the 40mph restriction at March station, be advised at what speed to run at to arrive at Ely North Junction with a clear path through the Ely area. Accelerate to 45mph or 50mph. On the approach to Ely station reduce speed to 40mph for the junction onto the Bury St Edmunds line.

As can be seen, CDAS can provide a much smoother speed profile, thus saving brake wear and fuel.