Most TOCs pay a trainee driver in the low £20Ks and on completion, when driving productively, pay the full rate. A few will stage the full pay over three years but not drastically, perhaps 4/5K between the new driver and the full rate. The only seniority pay to be found will be for extras responsibilities such as instructor/mentor.
A headline rate of £50K in four years depending on inflation will be on the low side of the norm across the network. Several companies today pay over £50K basic. Because of this companies are always trying to run depots without a full establishment of drivers. They offer inducements of time + a quarter to work rest days, the same to work a Sunday when they are outside the working week and some will offer an extra £50 for turning up to work a rest day. The unions are constantly fighting for full establishment figures so that drivers do not feel pressured into working days off with the attendant fatigue risks.
Personally I will work two Sundays and two rest days every four weeks, this pays for 6 hrs training towards my NPPL(M) after stoppages.. However, some of the lads in their early/mid 20s will work every hour they can to reduce London size mortgages and achieving a £1K net weekly average is not uncommon.
Part of the cause of this is the greedy TOCs that want to wring every last penny of profit out of the franchise before it is invariably lost to another company. They will provide the minimum of service they can get away with, employ the fewest staff they can, put up fares as much as they can and through higher salaries 'poach' qualified drivers from other companies so to avoid training their own as much as possible. Training costs are far more than an integrated course, MCC, JOC and TR put together.
Is it fair? No, but it is market forces coupled with strong trade unionism.