How accurate is the TS back pressure / exhaust limit?

[TrabantDeLuxe]

Here's something for you to consider.

How accurate is the implementation of back pressure in steam locomotives as implemented in TS2017? As far as I can see from experimentation, what TS does, is use a 'exhaust limit'. The exhaust limit is defined as the maximum steam flow rate (lb/hr) that can be passed through the exhaust system. For the purpose of this discussion, let us define the exhaust system as the chimney, blastpipe, and valve system. It seems that once you pass this limit, tractive effort is set to 0, forcing the player to decrease the cutoff (i.e. 'notch up' ). I figure most of you have noticed this before.

So here's a question for those of you lucky enough to have driven a steamer (are there any of you out here?), or those of you named Chapelon: Is this effect noticable in real life? I haven't got much to back this up, but it would seem to me that back pressure does not suddenly come in to play, instead slowly rising as speed (and as such steam flow through the exhaust) rises. I also don't think back pressure will take all the tractive effort away (that would require rather high values of back pressure), but instead, merely reduces tractive effort partly.

[SVRSam]

I've only driven a steam loco once on a preserved railway so never worked it hard enough to get near the limits seen in TS. But I seem to remember reading somewhere that to get to 126 mph Mallard was run at 40% cut off!