Add loads of friends and collect trains from their stations every day. Yesterday - I even managed to get a new loco (one of 4 missing) - so miracles do happen.
Sadly - the recent gem offer locos are almost impossible to get - and with decennial - most players got offered a hyperloop - so I would be surprised if all of them exist in game. I doubt anyne will ever get a complete museum again.
As for the XA Quad.... that was my bete noir - my last loco for a complete museum - way back when. A friend managed to get it for me - but cost me a fortune (happy to pay tho).
As for museum trading groups - the one I was in - was by far the best - at one point we had 20+ active members and over 90% of all locos in game. Sadly most of those have got bored and left, and the rest of us have pretty much completed museums - and only locos no one will have -so there is a brief flurry post update and that's it 🙁