Both rail and bus are heavily subsidized -- buses come nowhere close to paying for either their operating or capital expenses with fare revenue. It is probably true that if you broke down the WMATA budget and looked at the degree of subsidy per passenger mile on rail vs. bus, you'd get a higher number for rail than bus, but it's tricky to do an apples-to-apples comparison because the rail figure will include the cost of the stations, tunnels, and track, whereas the bus figure won't include the cost of the roads, but the government pays for the roads too, just not as a line item within the transit agency.

The reason to subsidize rail is that if you didn't, many people wouldn't take the train, they would drive or take the bus, and we don't have the road capacity for that. Rail is an exceptionally efficient means of moving lots of people with not very much space, and drivers and bus riders benefit from those people having been moved off of the roads in the form of reduced traffic.