Full Idea
It seems possible to define a train in terms of its carriages and the connection relationship, which would meet the equivalence account of abstraction, but demonstrate that trains are actually abstract.
Gist of Idea
Abstraction by equivalence relationships might prove that a train is an abstract entity
Source
Gideon Rosen (Abstract Objects [2001], 'Way of Abs')
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.9
A Reaction
[Compressed. See article for more detail] A tricky example, but a suggestive line of criticism. If you find two physical objects which relate to one another reflexively, symmetrically and transitively, they may turn out to be abstract.