If you set "slow speed" as a setting, you can then set this speed which the loco will not go above. This speed will be shown on the HUD, presumably so you don't wonder why you won't accellerate past 10mph or wonder why you can GET to 5mph when you should be doing 3mph to load properly
It's not quite so straightforward as it maybe could be since it involves a couple of things. I was going to explain it but Matt did that in the stream last night So over to him at 1hr 13 minutes. Spoiler: Slow speed control 66
The slow speed setting is used in the Aggregate Industries scenario on GWE, which is a good introduction to it.