A 9 volt 'transistor' battery, the square one, would do a good job for a while, particularly a lithium one, and especially a rechargeable one. As Chip points out, though, unless you can dial up and down on that voltage, as soon as you throw a switch, you get instant speed and a brief spinning of the drivers as the engine struggles to get up to the commensurate speed. Hardly a good way to run a train if you ask me.
Also, you would need enough batteries in parallel to get enough amperage to run a train of a decent size, especially with stiff truck axles, curves, and grades impeding forward motion...or rearward for that matter.
When I was building my current layout four years ago, I had a small diorama wired to a single 9v battery. It made the DCC sound system come on, but it wasn't enough to make the engine, a Lionel HO Challenger, begin to move down the 10" of track left in front of it. So, while you may have the volts, you won't have enough in reserve in the way of amperage to make the engine move....maybe just nudge forward a bit, but that's all.
-Crandell