This page includes links to additional cartographic resources on the web, along with suggestions of other institutions that may hold more local or more modern railroad maps.
As another federal administration that maintains historic records, NARA also holds a collection of government made maps which feature railroads.
For railroad maps of more local areas considering looking into the collections held at state libraries and archives.
Similarly, academic institutions may hold railroad related material as it pertains to their location and the history of the area.
Counties and towns sometimes collect and archival historic material related to the area. Occasionally societies are also created around particular subjects matter rather than a location. For example, the Baltimore and Ohio Railroad Historical Society aptly maintains material on the Baltimore and Ohio Railroad.
For more modern railroad maps it can be worth consulting the websites of other government agencies and institutions.