Why does it have to run on solid fuel? Fluids are easier to handle. Steam locomotives have traditionally run on poor quality oil. Recent examples are tho new and converted locomotives from [DLM][1] which run on light oil or diesel fuel. However, steam locomotives might use biowaste such as wood chippings or agricultural waste such as straw, though the energy density per unit volume is very low.

Apart from that, there is no reason why they should look much different from the locomotives of the late 1940s. It would be desirable if they could run equally well in either direction, which is primarily a matter of the design of the cab and fuel-and-water-carrying vehicle. Very large freight locomotives would, with advantage, follow the Beyer-Garrett configuration.

The great advantage of the conventional steam locomotive is that it is simple, with a relatively small number of components, of which the wearing parts can be designed for easy replacement. Also, in the railway environment, the boiler acts as an energy reservoir, providing a "buffer" between conversion of the chemical energy of the fuel into the potential energy in the steam, and thence to mechanical energy via the direct drive.

Boiler wear can be substantially reduced by using liquid fuel and effective water treatment at very high pH, and by keeping the machines permanently warm to avoid heating/cooling cycles. External combustion enables the locomotives to be designed so as to minimise the production of harmful wastes such as carbon monoxide, NOx and particulates.

The long preparation times referred to by some of the above commentators can be eliminated by internal or external pre-heating systems using oil, gas or off-peak electricity. 

In my view, it is time that this technology made a comeback at least on secondary routes where the density of traffic did not justify the capital costs of electrification.

Overall thermal efficiency these days is about 12% which is not very good, but waste heat can be used for heating the train. It is worth remembering that the most powerful locomotives in Britain are the class 68 diesel and the Duke of Gloucester, the last British Railways steam design.
  [1]: http://dlm-ag.ch