Actually all other forms of transportation besides trains get large subsidies.

Airlines like to suggest that the taxes they pay cover it but building, owning, and operating an airport is pretty heavily subsidized. As is the air traffic control system, most of the FAA and both anti-terrorism security and accident investigations.

Highways are also massively underwritten by the taxpayers. Heavy trucks lay on the vast majority of the wear and tear but their fuel taxes only cover a small fraction of the cost of building and maintaining highways. Then there is law enforcement and accident investigation.

Shipping is also heavily subsidized. The Coastguard is not funded by shippers and maintaining navigable channels and aid to navigation are all picked up on the US taxpayers dime. Inland you have all those locks and structures the Corp of Engineers has spent the last 50 years building to keep barges moving. All paid for by taxpayers money.

In fact of the major transportation systems the one that is least subsidized by taxpayers, and yet the one which taxpayers think they subsidise the most, are railroads.

Railroads are also inherently the most efficient of all methods of transport in terms of energy used/ton-mile or man-mile.