It's certainly not impossible that we might see a return to steam-powered railroads someday, but it probably wouldn't be with the traditional rod-driven locomotives. They pound the track too much for one thing, and they are maintenance-intensive.
I've been intrigued by the so-called steam motor locomotives shown here:

[www.dself.dsl.pipex.com]

[www.dself.dsl.pipex.com]

The design seems to have a great deal of potential. They used a compact 300psi watertube boiler which powered a six-cylinder steam motor mounted on the trucks just like a diesel's traction motors.
The steam-motor locomotives were built by Sentinel Waggon Works (so spelled) in England. These were the only road locomotives Sentinel built, but the company had a long history of building switchers, self-powered rail cars and road trucks all based on the same steam traction design. Interestingly they continued to produce the road trucks into the 1960s.
J