Have a fantastic time! I must recommend trying the VIRMs in the Netherlands, the ICMm's are also worth a go but the former are double-deck stock
Oh, and their DMUs, Class 3400s, are definitely worth a trip on. Can't precisely remember where they run but they do a branch to the west of Zwolle (I can't remember the name right now!) and a much longer run going the other way. Or at the very least they used to!
I must also recommend Liege-Guillemins to Luxembourg, a fantastically scenic line and the CFL Class 3000 locos are worth doing for sure. Same for the CFL 4000s, and the 2200s (double-decker EMUs) are also decent trains and worth trying out. Not such a fan of the 2000s, but on a higher speed and longer run they're probably better. However, although the seats are comfortable, expect Pacer-style seating on those!
In Belgium, the SNCB 18s and 19s are definitely worth a go (think UK 92s
), more so any 21s that are still going and 27s (certainly with the 27s, think UK 87s
) as well. More double decker trains on some of the stock, but the seats are not as comfy as the ones on the single-deck coaches. If you see any SNCB 55s working, they're worth watching as I thoroughly enjoyed their noise on idle at Brussels Midi, what they're like in motion I don't know. Sadly I don't think they touch any passenger trains. AM62s and their brethren, rather ancient EMUs now, are/should be getting withdrawn so worth sampling to see what they were like. However, should you be landed with a 'Beetroot' one I don't envy you, their interior is dreadful (in a literal sense)! Their replacements, AM08s, are much more passenger friendly.
Of course, it has already occured to me you're not likely to be doing much cranking, what with having your girlfriend in tow! Nevertheless, do enjoy the trip and I'm well jealous! Only ever done one Interrail, a 3 day Benelux one, but they are great fun and well worth doing!
EDIT: I don't know how rare it is for 3999 to make an appearance, but I've only seen it in action once myself a few years back. Lucky start for you!
), Paris to Mulhouse on TGV Duplex (double decker, love it!), which we were delayed by 20 minutes on, but weren't quite sure why as the announcements were only in French, then Mulhouse Ville to Basel SBB on a standard Loco hauled service (didn't see what it was though). Stayed with my family in Reinach, but didn't have a chance to go on the trams, as old fashioned as they look!