I am not sure how dynamic it is, but if you think of how your home broadband or mobile connection is managed, it is likely to be similar to that. Some networks impose a hard cap on speed, and this is what GTR and others used to do where you got 50MB (yes, that little) at full speed and then a capped amount after. In reality, the difference wasn't that great.
I am sure the Icomera website (
https://www.icomera.com) can help you find the answers to many questions, either on the site or asking them. There's a LOT of data (including downloadable spec sheets and technical data) on there from memory, although they are now under new ownership.
When it comes to video streaming, there are ways to limit the resolution and bitrate, and that might well be fixed. Again, without being on a train right now to check, I can't say. I don't stream YouTube, iPlayer, Netflix, Disney Plus or anything when on the train. I'm happy for data to send/receive emails, check forums and the like - all relatively low impact stuff.
I know GTR limited access to Drive, and I can't say I've tried a Teams chat or Zoom call. I am sure TOCs can make their own decisions on what to offer, based on the level of usage. An IC operator may want to offer more, and be willing to use more SIM cards to allow for a bigger overall 'pool' of data. Likewise, the operator will not only have the APs on the train to worry about maintaining, but also the type of modems they use.
Does the train support carrier aggregation? Can it aggregate with 5G on all bands? That's a HUGE difference. Someone with a Cat 3 or 4 LTE modem is likely going to never get above 50-70Mbps. With Cat 20 or above, they may get 300-400. With 5G and the latest Qualcomm modem, anything up to 1.6Gbps.
Now imagine a couple of SIM cards capable of pulling over a gig each, and sharing that. That's a game changer, and is purely down to some newer equipment installed.. not even needing new cabling to be routed inside the train or anything complicated.