the cross head can be designed to survive side-loads as it’s primary function, and can be bigger than the cylinder bore, as it is external, unlike the internal combustion engine’s little-end which has to fit into the piston. read more
An exception is the oscillating cylinder steam engine which can be double acting and yet has no crosshead. Locomotives Crossheads in a steam locomotive can be mounted either to one guide mounted above the crosshead or to two, one above and one below (called an alligator crosshead since it has two "jaws"). read more
The function of the steam engine is to convert heat energy into motive power, mechanical energy. A boiler generates steam by the application of heat, that steam is then expanded (losing its heat energy) into a steam engine which moves a load (locomotive, line shaft, pump, machine, generator, etc.). read more