[WallShadow]

The first time you make a scheduled run between settlements (cantonments/forts/etc), you have created an expectation of "civilization", a restoration of a measure of the pre-war ease of transportation and mobility. When word spreads of the run, passengers, merchants, farmers, manufacturers, and people with mail will flock to the railheads with their packages.
And once the pattern has been established and proven, public anticipation will drive a greater feeling of "civilization"--think "the Wells-Fargo wagon" in "The Music Man", or Sears & Roebuck's Catalog and the dreams it generated of items that could be had from far away.
A further result would be the extension of settlement, and hopefully, security, along the rail path from both ends. Concerned parties would be watching toward that direction and would probably institute patrols to ensure continued safe service. Urban Sprawl, but along an expanding transportation conduit.
Rail service would then become a Holy Grail for aspiring settlements located near existing rail lines, a bandwagon they could jump onto to boost their town's prosperity. You might see a small town scavenging disused rails and ties from one line to connect to a prospective "live" line to get in on the action.
Lots of adventure opportunities for players to assist various groups, ride shotgun on cargoes/escort passengers, get a vaccine to a town with an outbreak of disease, or hijack a valuable shipment (Firefly--"the Train Job"), etc.
The train service could also serve as a plot device--"He's coming into town on the Tuesday run--and he'll be gunning for you!", or as a medium for drama--"They all stabbed him, Inspector--it's the only explanation possible!". The _building_ of the rail lines may provide inspiration--two towns or business concerns racing to finish their connection first, with all of the dirty tricks, sabotage, and outright murder that could entail.