Hey Barry,
It adds engines totally fine with 3 TIUs. Totally solid. With 4 it works but intermittently. With 5 it just doesn't happen.... It always times out. As a work around I've been powering down two TIUs to add the engines then put them back after. Once the engine is actually on the engine list you can move it between inactive and active without a problem, it's just getting it added that's the issue.
On a quiet day when it's just me, powering down 2 TIUs will work, but when the club has lots of people running trains, that's not really going to be an answer.
Cheers!
~Adrian