You can't compare the Depot Driving you did during your training on Newton Heath with your Driver Trainer with performing the role of an MDD. I did that same training during the first 6 weeks of my course, and there is absolutely NO COMPARISON.
You didn't even scratch the surface of what an MDD role is about.
Mainline is more monotonous than MDD Driving - *Especially* if you only sign a few routes. For christ sake you driving all the time, some days up and down the same route for most of the day in the seat for 6 hours on same type of unit.
Now in a typical night at Newton Heath, you would have performed around 20 different shunt moves (there are well over 10,000 posibilities) around a busy complex depot, avoiding conflicting moves using hand-held radios. Performed ground shunting tasks, pulling points, handsignalling propelling moves. Shunting using radios or bell buzzers. Despatched trains off the sidings and dealt with depot arrivals. Fuelled and tanked, topped up engine oil and coolant levels on a few units. Carried out fault finding tasks probably about 15-30 times in one shift, dealing with all sorts of faults. Coupled & uncoupled over 10 times. Carried out a wide range of safety-critical comms about 50 times and listened to & acted upon over a hundred safety-critical messages on the radio. Walked over 8 miles. Re-positioned units on the fuel rig. Split units in half. Put split units back together again. Moved or assisted with a swinger unit or two. Repositioned units in the shed or the underfloor for maintenance. Raised the EBS, isolated the TIS, isolated/disolating batteries or the Engine, isolated isolating cocks, at least once per shift. Prepped different units, probably about 6 to 8. Got on and off different units all the time, the full range of traction. Marshalling units in the yard. Leaving room for appropriate types of units and arranging formations. Lots of problem solving as well. Thats without mentioning the team work, plus the amount of breaks you get if you want them. The fact that the job has a better balance of outdoor and indoor work. Then theres the relaxing moments to break up the action, the fact that sometimes you can be paid to sit on a comfy 156 unit for 20 minutes while it's being fuelled, or paid to hang around a knackered unit that takes forever to build up air. Every moment is different, you don't spend long on the same task, its what makes it great and makes the 11 or 12 hours feel like 6.
Hopefully that will enlighten you about the MDD role and that your comment "One day 35 metres down 3 Road, the next day 37 metres down 3 Road" being somewhat misleading. To someone who wants to drive trains and have a lot of variety, its well worth looking into.
We had someone leave NH for Mainline nearly 2 yr ago, saw him for the first time in ages the other week, says its so damn monotonous on the mainline and regret it. He only signs 4 routes.