I've never had trouble eating on board any food or drink I've taken on with me, nor ever had trouble with, eg, a sarnie or whatever I've carried right through the system to eat after arrival. However, there are in theory rules about not taking things (such as food) which could carry anything infectious and hence spread some disease from one country to another [though I'm a vegetarian and rarely travel with a chicken carcass], which is fair enough - and I think has historically applied in other situations too (like transit between the island of Britain and the island of Ireland).
Though in the same way that the re-established "customs checks" now operate in theory on arrival at Gare de Nord etc, but in practice no-one who looks like a "normal holiday-maker" etc is ever stopped, I assume the "health checks on arriving animal products" rules are also completely nominal.
But on-board eating/drinking your own stuff is not even a theoretical problem - as long as you don't cause a nuisance! (On a recent Brussels-London journey a loud guy in the seat in front of me had clearly been enjoying a bit too much Trappist beer before leaving - but of course if it's consumed before departure there's little to be done anyway...)