Help with A-B-A diesel engine set 20-2650-1

No.  No lash up.  All you need to do is connect the tethers.  I would advise swapping out the battery for a BCR2 (or at the very least a new battery) since it's over 15 years old.  The battery is in the lead A unit.

Thank you MikeH. Now at the risk of sounding dumb how do I know which is the lead A unit and which is the trailing unit? Bye the way I LOVE the knowledge on this forum and the willingness to share.

That set is from the 2006 Vol. 1 catalog.  On the MTH website, none of the FA sets have a manual link.  That is weird.  Here is a link  to an F7 ABA set from the same catalog.  It should be similar.  Alternatively, here is another link to FA ABA's from the 2010 catalog.

I believe the lead A will have a female socket to receive the tether.  Also, I'm not sure if it's the case on your model or not but the door diaphragm may tip down to reveal the smoke and volume pots.  The lead A will have those.  The trailing A will not.

Like this...

Perfect thank you so much!