Learn & Understand

Fixed Block to Moving Block: The Fight for More Trains Per Hour

The headway calculator turns separation distance into a minimum time gap between trains, and that gap is the master lever on line capacity. For a century the gap was set by a crude assumption baked into the signalling itself. Modern systems attack that assumption directly, and the difference between "fixed block" and "moving block" thinking is the difference between a line running at half its potential and one running near its physical limit.

The Fixed Block Wastes Space

Traditional signalling divides the track into fixed sections and allows only one train per section. To be safe, a following train must treat the entrance of the occupied section as the danger point, even though the train ahead may be far along inside it. The result is that a large, fixed chunk of empty track is reserved behind every train, space that exists only because the signalling cannot see exactly where the train ahead actually is. Headway is padded by the block length, whether that padding is needed or not.

The Brick-Wall Assumption

Underlying safe headway is a deliberately pessimistic idea: assume the train ahead could stop instantly, as if it hit a brick wall, and ensure the following train can always stop short of it. This is safe but wasteful, because trains do not actually stop instantly; a following train's own stopping distance overlaps the leader's. Fixed-block systems cannot exploit that overlap because they do not know the leader's precise position or speed.

Moving Block: The Gap Travels With the Train

Moving-block signalling continuously tracks each train's exact position and speed and communicates it, so the safe zone behind a train is computed in real time as just its actual braking distance plus a margin, and that zone moves along with the train rather than snapping to fixed section boundaries. The wasted reserved space collapses. Trains can safely follow much more closely, cutting the minimum headway the calculator computes and, directly, raising the trains-per-hour capacity of the same physical track.

Why This Is the Cheapest Capacity

Building new tracks is enormously expensive; upgrading the signalling to squeeze the headway is comparatively cheap. That economic asymmetry is why hard-pressed urban and metro lines invest in moving-block and communications-based train control: it extracts more capacity from existing infrastructure by removing the empty space that fixed blocks were forced to reserve. The headway figure is, in effect, a measure of how much the signalling still doesn't know about where the trains really are.

The braking distance that sets the moving-block safe zone comes from the Braking Distance Calculator; for the fixed-infrastructure version of the same capacity problem, see the Signal Spacing Calculator.