For the drivers (particularly Dieseldriver above):
What happens in your case when you're coming up for a crossing or other place to whistle and the handle comes off? Do you have to try and get as close to 20mph before the crossing? Let's assume it snapped before you could make any noise.
And secondary: what do you do with a fault like horn etc that develops en route? Do you stop the train and report to the signalman what has happened? Or ring him on the move? I believe trains have phones in the cab now.
And for the headlight: is there an indicator in the cab? Or do you find out when someone passes on the message?
Faults fall into 2 categories. Essential faults (brakes, doors/interlock, horn, headlight, radio, AWS/TPWS etc need to be reported immediately to the signaller (so either via cab radio or stopping at a signal telephone asap). Non essential faults (partial horn fault so at least one time works for example) have to be reported to the TOCs control at the first convenient opportunity. It depends on the severity but some drivers may see the next booked station a convenient opportunity to phone control, others may wait until they finish the journey etc.
Some newer trains have proving lights in the cab for the headlight. That should go out of the headlamp goes out. But normally you would be told that another driver/platform staff/track workers have reported that no headlight is shown at the front of your train.
To be slightly pedantic, trains don't have phones in the cab-it is strictly forbidden for any phones to be on in the cab. All trains do have a radio though which provides communication with the signal box.
If I was whistling for a crossing and the horn broke I'd bring the speed down to 20(as I would at any time it broke regardless) and look to check the crossing is clear. If it wasn't I'd place it into emergency and come to a stand.