STEP 3

THERE are no buffers allowed, so the track going up from B must link to that spare piece above it. This means the track in the second column has to keep rising: it can’t join up yet. Still, at some point, it will have to travel across to the right-hand side of the grid. In doing so, it will use up a fourth cell above point B — and we are allowed only four pieces of track in that column.