Based on the description from Cartographer’s locomotive - Cabin:

The living quarters of the cartographer. This was designed to be the observation car of an ostentatious Art Deco-styled locomotive from which the cartographer could observe the surrounding terrain.

It seems like not all the surroundings would be cartographed.
Also, the description in Cartographer’s Locomotive says:

Design for a vehicle bringing an explorer-type character on a long journe

I imagine the train tracks being in a not very explored area.