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TNT have subcontractors who deliver goods for them. These drivers use their own vehicles to carry out deliveries. TNT used to force these drivers to have the TNT livery on their vehicles, but around 5 years ago their stance on this changed. I assume it was something to do with the fact that drivers couldn't really do any work for anyone else with TNT emblazoned across their vehicles, thus technically making them solely employed by TNT.
I hope this explains why you don't see as many TNT vehicles around anymore.