Sooooo obviously a trick question.

First, train X was on the West Coast Main Line, traveling from Glasgow to London, so the distance was more accurately 399 miles, not 400.

Second, train Y never met train X, because train X disappeared.

You see, train X wasn't actually traveling slowly. Forty miles per hour was its average speed between 5 pm and 6:39 pm, when station logs officially note that contact was lost. The train actually stopped at about 6:06 pm, an hour and six minutes out of Glasgow, because a porter had discovered the body of one of the passengers--Professor Miles Throckmorton, of the University College London Institute of Archeaology--in the baggage car.

The body's condition was ... odd. Its eyes were shriveled and sunken--dry, funguous nuggets wedged in the back of gaping sockets--and its mouth was stretched so unnaturally wide in what seemed to be an expression of shrieking terror that the jaw had actually come unhinged. Also, the skin had somehow petrified. When the panicking porter tripped over the corpse in his scramble to escape the baggage car, a large section of its cheek crumbled away in sharp, stoney flakes.

The (needless to say extremely shaken) porter informed the conductor, who immediately ordered the baggage car closed and stopped the train to investigate for himself. He then called Glasgow Central at 6:18 pm, asking for instructions. When the station called back at around 6:25, train X did not respond. For the next twenty minutes, the station tried without success to contact the train, finally sending another engine down the line to find it, but there was no sign of train X. The Rail Accident Investigation Branch was called in and the entire 399 mile length of track, as well as the surrounding area, was thoroughly searched, but the train, along with its 83 passengers and crew, was never found.

A few interesting things about the case:

According to the UCL Institute of Archaeology, Professor Throckmorton was returning from a visit to their Doha, Qatar campus. An archaeological dig in the area had uncovered some unusual artifacts, and the professor was so amazed by what they had found that he took it upon himself to personally transport what he considered the most startling piece back to London. The only description of the artifact comes from Throckmorton's last e-mail to Professor Pamela Chastaine, a colleague at the University. "The fellow is astoundingly and disturbingly grotesque, carved with a precision and technical aptitude exceeding anything previously seen from this period. I'm not sure which possibility frightens me more: that it will turn out to be a masterful hoax, or that it will not."

No photographs of the artifact exist. Apparently, all the cameras used to photograph the find were exposed to some sort of powerful static or magnetic discharge, and the SD cards were destroyed.

It might also be worth noting that the conductor, in his last communication with the station, mentioned that the Professor's corpse was clutching a metal case, "hugging it so hard they're going to have to cut off his arms to get it loose."

And finally, the last trick in the trick question: if you've been following along and doing your math, you might have noted where train X was when Glasgow Central lost contact--almost exactly 66 miles from Glasgow and 333 miles from London.