To be slightly less concise -
Absolutely, yes. TUPE is law not optional (at least at the moment - elements within the conservative party have talked about abolishing it, but not sure that's official party policy)
Where 'an economic entity' (in this case a rail franchise but it applies for something like a council's dustcart contract being re-tendered) is transferred then all staff are entitled to transfer to the new employer on their existing pay, terms and conditions, and length of service (for things like entitlement to redundancy pay / protection from 'unfair dismissal')
Obviously casual staff and the like are then easier to sack than those on permanent contracts (although they do have some employment rights) And staff employed through agencies and sub-contractors may be in a different position
(although conversely, if a TOC re-tendered its station cleaning contract part way through a franchise, those cleaning staff ought to have the right to TUPE to the new contractor - although it would get more complicated if those cleaners do other work as well.)
But there are plenty of railway staff out there who got TUPEd from BR to one franchise company and have been transferred again a time or two.
The old employer may (but does not have to) offer staff the chance to stay with them if it has other work (unlikely in the case of something as big as a rail franchise), the new employer can't refuse to take individuals or groups of people on, and the staff can't refuse to be transferred - if you decline to transfer, it's legally resignation not redundancy.
There is no definitive time limit in law how long such protection lasts but an awful lot of opinion bandied about on this. It also gets complicated if the new employer wants you to move to a new base.
What is not so simple is any 'central services' roles at group HQ - if Windmill Holdings plc was to lose one of its rail franchises, it may find it needed less people at group HQ but those people didn't work solely for that franchise. That might be more complicated and result in redundancies.
(The above is based on personal experience and research - I'm not a lawyer so it's not "legal advice" - if you're facing TUPE I'd suggest seeking qualified advice, and your union's probably the best place to start.)