These are typical tank trailers of the 50s, both made to be connected to a two axle (4x2) tractor, in this case a 1940 to 1950 manufactured Autocar C-90

Here’s a typical early 1950s era tank trailer that would have been used into the mid 1960s with a single rear axle (4x2) tractor. Sheepscot Kit 95057 is a solid urethane casting with a styrene channel rear bumper and cast metal filler hatches. Tires are cast metal 11.00-20. The decals shown are custom made on the computer.
Other uses for a model railroad layout might be a fire department water tanker or a storage tank outside an industrial building.