Peter Corser wrote:

The half minute timing accuracy was down to the programme (sequence)
machines stepping every half minute. The timetables on the machines ran in
half-minute time (from 0300 to 0300 the next day) with 0 at midday - half
minute time was the most you could do within the limits of a computer
integer (8 bit - +/-32767 IIRC).

A good job they didn't have computers back in the 1920s then - the
Central London Railway used to run its off-peak trains at intervals of
one and seven-eighths of a minute. (Or 32 trains an hour.)
--
Joyce Whitchurch, Stalybridge, UK
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