Question:
Do rail-only units use rail cap to move? To retreat? Shouldn’t Rule 14F1 mention this? Is it mentioned elsewhere?

Answer:

Rail Movement: Everything that moves by rail in a friendly movement phase uses rail capacity. This includes units that can only move by rail. If these units were an exception, the rule would mention it.

Retreating: Retreats occur during combat phases, while rail movement and thus rail capacity expenditure occur in friendly movment phases. Thus, rail capacity is not an issue.

Source:
TEM 53