All change!
The 44t truck having been unavailable for several years, USA Trains have actually just had a new batch made. I'm importing a bunch into Europe, so I'll be updating my website first to 'expected', and then with a price etc. once customs have done their thing and I actually have possession of them.
Not that I want to loose any sales, but clearly anyone who's in the US already will find going to USAt themselves much faster/cheaper than double shipping them across the Atlantic.
I believe current availability is blocks, motors and both kinds of spare axle. The coupler mount is a separate part, and not currently available, though there are rumours for later this year/next.
In terms of which block fits what :
44t - has its own unique block and coupler mounting system.
NW2/S4/mightymoe - share the same type of block, but with different cosmetic sideframes to represent various 8ft wheelbase switcher trucks.
Everything Else (that's on four wheel trucks) - all use the same 94.5mm wheelbase unit with cosmetic bits representing EMD Blomberg trucks.
Given that the 44t version is available again, it's probably a moot point, but internally, while they obviously share a design heritage, there are precious few major parts in common - there are five axle types across the three sizes, three different motors (I believe the 44t and NW2/S4 share the same 'core' actual motor, but they use different versions, the NW2 has longer shafts and different worm placement to suit its longer wheelbase), etc. Some screws, pickup sliders/springs and traction tyres are about the only common spares.
Jonathan
g-bits.co.uk