Trains are land units with capability train. They can't be loaded on land units, but they can be loaded on ships, subject to the usual restrictions for non-light and heavy land units.
Unlike other units, trains can enter a sector only if it has (operational) railroad track.
If option RAILWAYS is disabled, a sector has railroad track as long as its rail infrastructure efficiency is non-zero. Train mobility cost depends on rail infrastructure efficiency (see info Mobility). Spy and satellite reports show approximate rail infrastructure efficiency in column rl eff.
If option RAILWAYS is enabled, all highway-like sectors are railways, and the track is operational as long as the sector is at least 5% efficient. A sector is highway-like if its mobility cost at 100% is zero (column mob cost in show sect s). Operational railways additionally extend track into adjacent sectors that are at least 60% efficient and owned by the same nation. Sector selector track gives the number of operational railways within one sector range. Spy and satellite reports show the presence of track in column rl eff. To visualize your railway network, try sect # ?track#0.
See also : LandUnits