Gas turbines and reciprocating engines have similar thermal efficiencies. the big advantage GTs have over reciprocating engines is weight, both in the engine frame itself and in the auxiliaries. A GT is air cooled and provides its own cooling air, and it has only two bearings (or three in some cases)that need lubrication. Those are huge advantages for an airplane obviously. It is advantageous at all for a freight loco which needs a certain weight for adhesion? I doubt it.
GTs can not be idled for any length of time, mainly because emissions rates go up exponentially as load is reduced. In stationary practice 50% turndown is considered highly advanced and requires a number of added features, multiple rows of inlet guide vanes, variable burner geometry etc.
for ships, planes and stationary plants the load profile is start up, go to full load, stay there for a long time , shut down. this is a mission GTs are well suited to. can train trips be adapted to the capabilities of the GT? I dont think the GT is particularly adaptable to train operation.
GTs and recips can both benefit by combined cycle. The deployment of the combined cycle the HarryG describes is how gas fired GTs took over the stationary power industry from fossil steam plants over the last two decades. that raised the thermal efficency of a gas fired GT from 25% to 45%.
Most diesel ships have heat recovery boilers for heating and cooling the ship, and there are many, many combined heat and power stationary plants with either recips or GTs providing exhaust heat for boilers as well as electric power.
the boiler , water cycle equipment steam turbine are more massive than the GT however, and only account fo about 1/3 of total output, so its hard to envision how this could be adapted to a locomotive.
