There are three places you can find a Pin button for trains and journeys.
A button at the bottom
At the bottom of the ‘Train details’ and Journey screens, there are Pin buttons.
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Tapping on the info box
You can also tap on the info box at the top of those screens to bring up a menu with a Pin button.

Dragging open a Pin button
On the results screens, if you drag from right to left on a row, it will reveal a Pin button.

Unpin buttons
When a train or journey is pinned, the Unpin buttons will be in the same place the Pin buttons were.