The big RRs have usually required a diesel for years, primarily for dynamic braking - which is probably what you heard.
The other purpose for the diesel is in case the steamer 'breaks down' they can clear the mainline. I'm not sure what sort of failure they think won't result in an immobilized steam locomotive - throw a rod and it's going NOWHERE - but management convinced itself it was needed.
Ironically, the diesel has been the cause of the only major failure: the 844 steam locomotive - the last steam locomotive delivered new to the Union Pacific, never retired and 'in service' since the early 1940s - was pushed several miles by an unmanned, malfunctioning EMD SD70 stuck in run 8 (full throttle) while a number of nervous company executives and local dignitaries were on the train. It was stopped after considerable drama, but not before flat-spotting 844's drivers - which resulted in partial disassembly and the parts being shipped to 2 small specialized shops for reqairs. I don't know the cost, but it probably ran 7 figures!