Trains are simply an example of Mustelization, a similar phenomena to Carcinization where life-forms convergently evolve weasel-like traits, instead of evolving crab-like traits.

the long, flexible 'body' and short 'legs' are peak Weasel, way more than Crab. Perhaps trains are a combination of the best features of both, the spinal flexibility and body length from Weasels and multi-legged stability and armored exoskeleton from Crabs.