Prince Karl

In the short story The Girl in the Train, Prince Karl is a member of the royal family of the Balkan kingdom of Catonia. He wanted to marry his cousin the Grand Duchess Anastasia of Catonia. The Grand Duchess preferred to marry Lord Roland Gaigh. Karl and other members of the Royal Household came to England to attempt to frustrate the wedding. After the Grand Duchess evaded them, Karl thought she was in the company of George Rowland on a train to Portsmouth. He and the chancellor Stürm tracked down Rowland at a hotel there and demanded to known the Grand Duchess's whereabouts. George denied all knowledge of the Grand Duchess. Karl attempted to tackle him whereupon George floored him with a jujitsu throw.

Karl was described as a " tall, somewhat pimply young man, with a blond Teutonic cast of countenance which was not rendered more attractive by the fierce scowl which he wore...."

Portrayals
In the episode The Girl in the Train of The Agatha Christie Hour, a TV film adaptation of the story, the part of Prince Karl was played by Glyn Baker.

Trivial
It's never explained why Prince Karl and his companion called George for 'Lord Rowland', however considering that George's surname sounds almost identical to the first name of the grand duchess' fiancée Lord Roland, it's fair to assume that they thought George was the grand duchess' secret fiancée. They even pointed out that he signed himself as G. Rowland. So they could have thought that the G was for Gaigh. That would depent on whenever Prince Karl or Stürm have met Lord Roland Gaigh or not.