Hitchcock really comes into his own with this film. It's brilliant suspense, opening with the tracking shot down to the model of a town is totally suggestive of the break from his more classic shooting techniques. It was a model for later Hitchcock success and the standard of a classic crime epic mystery. This film also follows the notion of laying out the mystery upfront for an audience to get entangled with the intrigue and be as tense as the characters are trying to figure out what's going on rather than just shocking people with surprises.
The trope of mistaken identity and the mystery of a woman suddenly disappearing is so compelling in a psychological way. It draws you into the mindset of the main woman who believes she may be going crazy by thinking she saw this woman who suddenly disappeared.
I believe this is one of Hitchcock's greatest films. It's inherently thrilling, very simple in nature too, confining the story to a train car and it's journey. There's something theatrical about that premise. The dialogue and scenario are highly engaging and the story builds and builds in tension until the final shootout at the end when the train is stopped. This movie definitely shows off Hitchcock's ability to weave a thrilling mystery that hooks you and taps into your inner psychological mechanisms to get you to be invested in the story as it unfolds.