In the UK the rail traction motors are virtually all 4-pole. I can think of only one locomotive type which has 6-pole, everything else that is dc (locos, London Underground, all the EMUs) is 4-pole and has been for decades.
With an increased pole count the armature frequency does go up, so more expensive laminations are required but conversely the thickness of the frame iron can be reduced assuming the mechanical strength is still sufficient. Also the total amount of copper on both the armature and the field coils can be decreased because of the shorter end windings. However, the manufacturing cost will go up (all those extra pole bricks and armature coils) and also because the number of brush arms = no of poles on a lap wound machine, the brush arm accessibility for changing brushes under the vehicle becomes a problem. The commutator can be made shorter though.
So for typical traction motor ratings I think it comes down to the lower pole count being cheaper. Assuming you are talking about a traction motor, I can only imagine that an 8-pole was used for some marginal weight saving and reduction in size.
I don't know about industrial dc machine practice.
