Neil,
I´m currently at my girlfriend´s, it´s 36 degrees centigrade, the charcoals in the barbecue grill are heating (as are a couple of Bratwursts on top of it), and officially, I´m browsing the web for some porcelain cats by Rosina Wachtmeister as a gift for her...
...but at home, I have a nice book on the Orient Express which I promise to check for answers to all your questions tonight

The length of the Trix coaches is scale afaik from the top of my head, and iirc, they were rather expensive because they feature interior lighting and even small lamps on the table of the dining cars that were illuminated, although I´m not really sure on that (I must admit).
The locos were indeed interchanged; typical German locos included the class 18.3 (Badonian class IVh), available from Bachmann-Liliput, class 18.4-6 (Bavarian S3/6), available from Roco, Liliput(Vienna), Rivarossi(Italy), and Trix (which lacks the detail of the others imho), and the class 03 (available from Fleischmann). Another typical German engine for the Orient Express would have been the class 17 (Prussian S10) variants, as available from Roco or Fleischmann (€€€€€ - sigh...). All this info concerns the past-WW1-years; I´ll try to obtain more info for earlier years later...
my food is ready, I have to get back to the garden barbecue.