It's also a matter of design. If it has to carry it's own boiler and firebox (or some other method of producing steam as it moves) it's going to be heavier (and a lot more dangerous. Boilers that are built wrong have a tendency to explode, with disastrous results). The railroads, however, had several designs of locomotives primary used for yard switching that were, essentially, giant tanks for storing high pressure steam. When the steam pressure got low, they would pull in and top off from a stationary boiler somewhere in the yard. That kind of design wouldn't require more than a high pressure vessel suitable to hold the required pressure of steam, and would make your design lighter, as it would remove the weight of the boiler, firebox and fuel.
As CP said though, good luck.
