Signal spacing usually gets treated as a downstream detail — something resolved once the alignment and the timetable are already fixed. By the time it reaches a signalling engineer, most of the decisions that actually determine feasibility have already been made elsewhere.
That ordering is backwards. Signal density sets a hard ceiling on how many trains a section can carry per hour, and it interacts directly with braking curves, gradient and approach speed. Treat it as a late-stage detail and you inherit whatever throughput the geometry happens to allow.
The trade-off nobody names out loud
Denser signalling buys headway at the cost of more equipment, more maintenance points and more failure modes to design around. Sparser signalling is cheaper to build and maintain, but caps capacity in a way that is expensive to undo later. Neither answer is correct in isolation — it depends on what the line is actually meant to carry in ten years, not on opening day.
A signalling plan is a capacity commitment written in concrete. Change your mind after it is built, and you are not editing a document — you are re-laying track.Field note, systems review
This is the case for bringing signalling into the same room as timetabling and civil design from day one, rather than treating it as a specialist function that receives a finished alignment. The earlier the constraint is visible, the cheaper it is to design around.
