I've just removed the previous 24 hour training limit for staff and drivers. It should be the case now that they can be trained after each race that you participate in. That means you could train them just before a race and again just after it, but only if you participated in the race.
This gets around the need for enforcing a 24 hour threshold as the races you participate in will be regulated to one a day. But it enables you flexibility to train whenever you want between races.
It's a long story about why that limit had to be enforced in the first place, to prevent league-hopping antics some people were using to get extra training. With this method we should now have the best of both, no exploits and freedom to train when you want.