Ignore those rail salaries. The average is much much lower. Yes, it's possible for a signalman to earn that if said signalman is in the highest graded (9) signalling centre, is on London allowance and is putting in a maximum allowed 72 hours per week.
The same applies for the other jobs mentioned.
Source was probably the Daily Wail cherry picking it's facts again. An awful lot of signalmen in smaller 'boxes are on less the £30K per annum.