Yes, please map what is on the ground. If there are any visible traces that a road or track or something were a railway, then it's fine to add former railway tags. But if there is absolutely nothing on the ground, then there should not be any tagged features, for example over completely rebuilt areas, intersections and such. You can add an empty line for a historic railway relation for railways that still have visible elements somewhere else, but the line itself should not specify any tags.