Normally I wouldn't care how you use a mod, but many people use this mod not only for the convenience, but also the UPS savings, and making it work like you're saying can be a pretty big UPS hit.
It can be provided as an option.
Some of us -- me included -- want to use this only for simplifying things. As a replacement for the venerable "Bulk Rail Loader" mod that is still not working properly with 2.0.
So if desired, player can choose "instant loading" or "gradual loading". The former if wanting to save UPS, the latter if wanting to reduce the OP-ness of instant loading.
My point is encouraging one to not feel it that OP, nor that unrealistic, as it's far more realistic than putting the exact same cell that you'd put in a rocket in a train and "just having it work."
Given the small lift weight, extreme shelf stability, complete lack of piping required, and the derivation of the fuel from a solid-fuel mix, it seems more like we're using effectively super-sized model rockets with a solid fuel type, or fuel cells. Using such rocket fuel in a train would not just be ineffective, it would
very likely explode the train from the pressure. Rocket fuels are designed for specific impulse, not for heat, which barring some serious retooling of the train, seems unrealistic to just hot-swap with coal.
It'd deffo be nice as a feature to have controlled loading speeds, not just from a "oh no OP" standpoint but for the sake of more granular timing, just critiquing the constant worry of "OPness" or "unrealistic"ness that pointlessly gamifies things that exist in real life just fine, when there are far more 'gamey' unrealistic mechanics in the game, as many of my favorite mods have been made needlessly complicated over worries of such things from third parties.
Semi related - mayhap to help avoid UPS issues, rather than a transfer speed, you could use a delay before trains are loaded/unloaded. This may help to still sufficiently simulate loading/unloading while still remaining "fair" as well as avoiding unnecessary calculations?
In real life, the delays involved with these are similarly more of just that - a delay while trains are coupled to the track, or anti-dusting measures are taken.