Sterling Engines are a type of heat engine , that use pressure differential caused by a hot interior and cold exterior . They only function as long as the heating core is warm . But speaking as a mechanical engineering student , Steam is pretty hard to beat in terms of general power efficiency . There are more efficient ways to do external combustion than steam pistons like turbines but pistons can provide reliable torque at most speeds and can go backwards . Turbines can ' t reverse and only produce effective power at one optimal speed . There are other working fluids to use , like air pressure , glycerin or ammonia , but water is cheap , easy , reliable , nonpolluting , nontoxic and has a high specific heat capacity . That ' s why people used it so much , and people would ' ve continued using it with the forthcoming optimizations of the 1940 s if Diesel wasn ' t immediately superior in every way . Diesel became cheaper in the 1940 s making it more cost effective than coal , require seconds to start instead of hours of firelighting , and could be operated by one person , and more reliably with fewer moving parts . That all said , steam locomotives are still used today . It ' s impossible to beat the ease of access to fuel just burn something like wood and add water . And because steam is external combustion instead of internal , engine performance is independent of outside conditions like weather or pressure . Low pressure on mountains is a problem diesels and dieselelectrics can suffer from . I would recommend getting more creative with the application of expanding fluid pistons . You don ' t need to make everything a differentlylabeled steam engine . Check out fireless locomotives or compressed air vehicles .