A growing phenomenon in the airline industry is for high speed rail trips to be sold by air carriers in the same manner as airline tickets, so that they can feed multimodal airports like Charles de Gaulle Airport and Frankfurt Airport. The train trip is given a flight number, and the station is given an airport code, so that your jaunt on Deutsche Bahn, SNCF, or Thalys looks like a flight on Finnair or American Airlines. One caveat to this is that the stations have to be fit into the already-congested list of airport codes, so they often have outlandish TLA's attached to them. Here are a few I've discovered: /msg me if you've got more.

BNJ Bonn
MHG Mannheim
QDU Dusseldorf
QFB Freiburg
QKL Cologne
QJZ Nantes
QLJ Lucerne
QLS Lausanne
QQK London (Kings Cross)
QQM Manchester
QQN Birmingham
QQP London (Paddington)
QQU London (Euston)
QQW London (Waterloo)
XDB Lille
XKL Kuala Lumpur
XOP Poitiers
XRF Marseilles
XSH Tours
XYD Lyon
XZN Avignon
ZAQ Nuremberg
ZBP Baltimore
ZDH Basel-Mulhouse
ZDJ Berne
ZEP London (Victoria)
ZFJ Rennes
ZFV Philadelphia
ZLN Le Mans
ZMB Hamburg
ZMU Munich
ZTF Stamford
ZVE New Haven
ZVR Hanover
ZWI Wilmington
ZWS Stuttgart
ZWU Washington, DC
ZYP New York City (Penn Station)
ZYR Brussels (Midi)

Thanks to gn0sis, Apatrix, and trainman for adding to the list

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