Train UI should contain all of the information available in the train tooltip

[Teod]

Fuel: Clearly stated in the tooltip, but if you click you can't look at it, unless you're standing right next to it.
Cargo contents: Tooltip has the exact item types and amounts. Click at best gives a green bar, and only if the train is currently at a station with a cargo condition. There are no numbers or item types.
Speed: Tooltip gives you current and max speed, as well as acceleration force and mass (not as convenient as a straight up acceleration would be, but it's something). Clicking gives nothing.

For stationary entities this kind of difference isn't bad, because clicking and hovering takes the same amount of effort from the player. So it's okay to keep them as sort of two halves of the interface - the one you interact with (click) and the one you just look at (tooltip). But trains move, making it at times really hard to consistently keep mouse over them, so any information you want to show to the player has to be available under a click as well.