There were three undersea tunnels. The one used by the European Special ran mostly freight trains. Muggle passenger trains were very fast, and if this old-fashioned "steam train" shared a route with them, accidents would be all too easy.
The faster they were, the less delicate instruments could withstand nature's battering. The salty air in the undersea tunnels corroded components, so those old-fashioned, roughly made, slow freight trains would never be completely retired.
Trains did not run themselves; they needed drivers. Human vision was better during the day and impaired at night. High-speed trains generally did not run at night; the ones that did were always slow.
In 1754, British Minister for Magic Hesphaestus Gore and the Italian Minister for Magic supported a famous broom race. Contestants had to ride flying brooms from Aberdeen to Rome, and to avoid drawing Muggle attention, the race was held at night.
Impatient people would find riding such a slow train agonizing. Why pay several times more than the cost of a plane ticket and spend several times as long on a train?
Early trains had no bathrooms. A journey from Paris to Istanbul, crossing the European continent, took two and a half days. Going without a bath for two days was still bearable.
But people with the leisure to ride such a luxury train generally did not mind spending a little extra time. The European Special now had bathrooms as well. The coal-and-water carriage that had once been little more than decoration had finally found a use, supplying passengers with hot bathwater. The compartments even had private lavatories, and a stick of incense burned above the washbasin, giving off a captivating "scent of money."
The carriage was made of mahogany, with thick Persian carpets spread across the floor. Two sofas stood by the