In traditional train control, the railway is split into control "blocks" with signals in each one. Blocks are sized to allow the heaviest or fastest trains to stop fully within them. That way if a train is stopped in the next block, the following train will always have time to fully stop before reaching it. Careful tuning of block spacing is needed; if they are signals are placed too close together, train speeds have to be reduced so they can still stop in time, but spacing them out further means the trains are also spread out and the route capacity drops.
SelTrac automatically maintains headway between vehicles through a moving block system. In this system the start and end points of the blocks are not fixed, and move along with the trains. This allows the central control system to calculate a point on the track where every train can safely move without further instruction—in a fixed block system this would be the next set of signals, but with SelTrac it is constantly being updated.
In theory this system could eliminate the "brick wall" criterion and allow the trains to run as close together as the communications speed would allow, but in practice a further spacing is typically imposed, 50 m in the case of the Docklands Light Railway for instance
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