I'm no expert by any means, but here are a few tips:
1. modifiers. there are 50-ish of them and each of them serves different functions. i find boolean, mirror, solidify, array, and bevel to be extremely useful. Don't forget to apply them once you're happy
2. start with something small, like a flat car, or a shunter.
3. use path for stuff like cables and hoses (shift + a > curve > path. bezier curves should also work well)
a few controls you need to now:
1. Middle click (alt + lmb if you don't have a 3-click mouse) to rotate the camera. hold shift to pan
2. Numpad . (dot) to center your camera on an object
3. Numpad / (slash) to isolate the selected objects (useful when focusing on small details on a complicated model, or can be used to see obstructed objects)
4. Shift + A to add objects. X or Delete to delete objects
5. advanced editing can be done via Edit Mode (press Tab, and you should see a menu for switching between modes)
6. join multiple objects using Ctrl J.
7. E to extrude and creates a new segment. use Alt + E to see more extrude options
8. G to grab, R to rotate, S to scale. you can also press either x, y, or z to constraint it to that axis, you can also type numbers. say i want to rotate a cube on the x axis by 45 degrees. i can just press, r, x, 45. you can also press - (negative/minus sign) to rotate it the other way. you can use Escape to cancel this action.
9. save often
10. Inset using I. useful for creating windows/holes within an object.
11. Use B to select things using click and drag. Use C to select things by clicking over them (kind of like drawing), useful for a lot of objects in one small space.
12. Shift + Click to select multiple things.
That's all I can think of.