A, D - Increase/decrease throttle or steam engine regulator
W, S - Reverser
E, Shift+E - Gear up/down
Apostrophe, Semicolon - Increase/decrease the train brake
Square brackets - Increase/decrease the locomotive brake
Period, Comma - Increase/decrease the dynamic brake
/ - Handbrake
Backspace - Emergency brake

T, Shift+T - Open doors/Load/Unload/Refuel (depens on your train and position)
H, Shift+H - Cycle headlights
J - Secondary lights (some locos)
K - Tertiary lights (some locos)
L - Cab lights
V, Shift+V - Windscreen wipers
U - Toggle blinds/glare panels
Z - Start/stop engine
P, Shift+P - Raise/lower pantograph
Q - Alerter
C - Acknowledge guard
K - Toggle DRA
X - Sander
Space, Shift+Space - Horn (high/low)
B - Bell

Tab, Ctrl+Tab - Ask for permission to pass red signal ahead/behind
G, Shift+G - Change switfch ahead/behind
Ctrl+Shift+C - Couple/uncouple rooling stock
T, Shift+T - Open doors/Load/Unload/Refuel (depens on your train and position)
Ctrl+-, Ctrl+= - Change cab

1 - Cab view
2 - External front camera
Shift+2 - Head-out-of-cab camera
3 - External back camera
4 - Trackside camera
5 - Passenger view
6 - Front coupling camera
7 - Overhead camera
8 - Exploration (free) camera
9 - Toggle 2D map
Left arrow, Right arrow - Alternative cameras for the current view
Ctrl+, Shift+, Arrow keys, Mouse scroll wheel, Holding RMB - Various options for adjusting your view

Esc - Pause menu
Ctrl+Q - Quit menu
Ctrl+E - Enter world editor
F1 - Task info
F2 - Save menu
F3 - Small HUD (on/off)
F4 - Full HUD (on/off)
F5 - Technical HUD
F6 - Toggle labels
F7 - Toggle rail vehicle number
F8 - Toggle destination overlay
F9/Shift+F9 - Reload assets
Shift+Z - Show framerate
Ctrl+S/F12 - Take screenshot
Ctrl+Shift+1, Ctrl+Shift+2 - Increase/decrease braking difficulty
Ctrl+Shift+O - Toggle Oculus Mode

Additional controls for steam trains.

J - Small ejector on/off.
C - Cylinder valves on/off.
F, Shift+F - Open/close the locomotive fire box.
R, Shift+R - Increase/reduce the rate the fireman shovels coal.
I - Turn on/off the live steam injector.
K, Shift+K - Increase/reduce the flow of steam to the live injector.
O - Turn on/off exhaust injector.
L, Shift+L - Increase/reduce the flow of steam to the exhaust injector.
N, Shift+N - Increase/reduce blower.
M, Shift+M - Increase/reduce damper.

Specific controls in some DLC's.

Shift+D - Hold brake
Shift+P, Ctrl+P - Raise/lower 3rd rail shoes
B - Depot whistle
Shift+Num Enter, Num Enter - Toggle/acknowledge Drivers Vigilance Device (DVD)

I - Switch to DC power mode
O - Switch to CTRL power mode
C - Signal buzzer
B - Open/close coupler hatch
Num Enter - Reset Drivers Safety Device (DSD)

Y, C - Increase/decrease AFB
Shift+Num Enter - SIFA Toggle
Num Enter - SIFA Acknowledge
Shift+Ctrl+Num + - LZB Toggle
Ctrl+Num + - LZB Automatic Brake Intervention Toggle
Ctrl+Num Enter - PZB Toggle
Page Down - PZB Acknowledge
End - PZB Release
Del - PZB Override