I haven't been in the business of procuring trains for many years, but unless the last issue of the procurement rules changed the situation dramatically, TOCs are required to go through a competitive tendering process as if they were state owned; this is because they operate under special/exclusive rights granted by the state. However, ROSCOs aren't required by law to undertake competitive tendering exercises.
It's difficult to generalise, because a tendering exercise for trains can be undertaken in many ways. For example, a TOC could just say it wanted to lease some MUs for a particular period, or it could - having gone through a tendering exercise with financiers - issue an invitation to tender to purchase trains in which title in the trains and the obligation to pay for them would lie with the financier, or a TOC could issue an invitation to tender to procure trains, then go through a financing tendering exercise, and then novate parts of the procurement contract to the successful financier. Simple !!!
It's also necessary to know details of procurement contracts, and with whom certain benefits rest; for example, although a TOC wouldn't be interested in owning a design for an EMU, it might want a license to use parts of the design so that it can procure EMUs from someone else to work in multiple with those it is initially purchasing - say EMUs from CAF or Siemens or Stadler to work in multiple with an existing fleet of Electrostars. Alternatively, it might have stated what type of coupling was required on its initial fleet, so that it wouldn't have any problems in then procuring more trains to work with them (think how Turbostars can work in multiple with Pacers/Sprinters/158s, and I believe this requirement was laid down by Porterbrook before they were prepared to finance the initial 168s for Chiltern)
TOCs have two ways of evaluating tenders; either on the basis of the lowest price tendered, or by determining which is the most economical advantageous offer. For the latter, the evaluation criteria has to be made known to bidders, and it is legitimate to consider costs associated with staff training.
When someone like Arriva was procuring trains for northern, it would have been sensible to include options in the contracts it entered into with CAF; it should also have included a provision to novate the options to a successor franchise (although this isn't essential - options could be transferred using a transfer scheme made under Major's Railways Act if the DfT so desired).
Note that procurement rules apply to all procurement exercises where there is financial consideration; therefore they apply to leases/hires as well as purchases. Moreover, if one TOC wants to hire in trains for a short period - such as how XC used to hire in HSTs for summer Saturdays - these are also covered by procurement rules if the financial threshold is exceeded.