I wonder why the Underground opted for the single bell - two "dings" seems a bit safer than one as it reduces the risk of an accidental signal being given.
I occasionally wondered that - although I was led to believe that there was an interlock so that the bell circuit wouldn't work if the 'pilot light' (that showed on the guard's panel to show that all doors were closed) was out.
I also heard that at one time, the starting bell was wired to ring in all cabs, which led to a few 'near miss' incidents at stations where trains were close to each other, allowing the driver of one train to hear the starting bell in the rear cab of the train on the adjacent line.
It also meant that an Underground guard couldn't give a 'stop again' signal like a BR guard could - if the guard was aware that something was wrong, or the pilot light went out after the train started, their only option was to 'pull the handle down' (use the emergency brake handle.)
Does seem strange, especially as a London Transport bus conductor used two tings to start a bus. A single ring stops a bus (usually)!
When buses had conductors, didn't one ding mean "stop at next stop" and two mean "OK to start"?
yes.
one bell generally means 'stop at the next stop' (although could also be used to mean 'cancel the start signal')
two bells to start (although some conductors and their driver would get used to starting on one bell)
three bells (not all operators used this) usually meant 'start, but bus is full - don't stop at any request stops unless you get a bell from inside the bus'
four bells, or two double rings (even fewer operators used this) could mean 'a vehicle is trying to pass, pull over if possible'
four or more bells, or as the London Transport rule book put it 'rapid succession of rings' meant emergency stop.