No experience of moving locos, but getting machines out of cars or vans isn't too dissimilar. I use a two stage process: and engine crane with a boom that can get in the back of the vehicle, remove the vehicle (usually easier and safer than moving the crane under load) lower machine onto a trolley and position under and overhead (gantry) crane that can lift from the trolley to ground level. The gantry I use is made from pallet racking, decent stuff will cope with well over a ton. The gantry could easily be arranged to cover both the track and an area for the trolley. For trailers you shouldn't need the engine crane because there's no roof in the way. The gantry crane can be electric if you prefer, though I find I get more control with the old fashioned chain type. Equally an engine crane is just an hydraulic ram with bits, if you really must avoid manual effort replace the cylinder with one that can be driven from an hydraulic pump power by a motor with valves for control - like a JCB! provided you've got firm ground or even better hard standing none of this should require excavation.
Probably not quite what you wanted, but hopefully some food for thought.
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