Railroads have many different reasons. I know that in 30 years ago, they were far more concerned about the steam locomotives than the passengers. Now days, it is usually the passengers (and the lawsuits) that are the greater concern.
There are many reasons for the diesels. Here are two examples I've personally been there for:
-leading up to the NRHS convention in Huntington, WV...the 1225 was to make its first major foray outside of Michigan and meet the veteran excursion engine #765 at Lima, OH. They were then to double head from Lima to Huntington. The 1225 was 4 hours late in arriving at the Rob Ave. yards in Lima. She didn't have a speedometer, and so they had to quickly round up a pair of diesels to make her legal (or at least kosher for CSX). The d-evils were then dropped in Lima and the 765 took the point with her speedometer...they put on an awesome show from there. Sadly, the 1225 had additional troubles while there and the 765 had to lug her dead plus a 34 car passenger train through the mountains (any surprise that an NKP S-2 was the first locomotive to defeat d-evils head-to-head?)
-I had the pleasure of riding the 30+ car passenger train behind the 261 through the New River Gorge. It was an eventful day. Thanks to the my father's long time friend Tom Stevens being the engineer that day, I received a cab ride for the photo run by. I then had the pleasure of riding the tool car and watching the 261 turn on the wye near Hinton. Tom mentioned that the 261 was far slipperier than the 765, and we really saw it as she was nearly unable to make it around the wye. We then returned to Hinton to pick up the train. Then we had problems. I don't recall what they were...I think it was an airbrake failure on the 261...and we had to wait for a pair of d-evils to arrive and couple aboard.
My last experience with d-evils and steam was a couple years ago when the 765 had some break-in runs on the PRR between Ft. Wayne & Van Wert. The local road, so generous in the terms to allow her on their rails, also supplied a dynamic brakes equipped diesel to pull around. The dynamic brakes were used to simulate heavy trains and give the berk a workout.
Railroad managements change over time, and different policies have existed. Some roads insist on their engineer being at the throttle, some don't...some have had previous bad experiences with steam and insist on diesel backup. Let's not use track of how real railroads used to have many locomotives hot and ready to step in if needed. The NKP used to use two PA-1s on their top passenger train for if one failed...and I've heard stories from engineers about losing one of their modern diesels while out on the line. I can certainly understand their fears about fowling the line with a breakdown...something they wouldn't trust even with a lone GP-40.
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I can't wait to see #20 rolling again. I can't get over what a wonderful job is being done on #20. WoW! I'd sure love to see her go for a test run on the EBT...you know, a break in run, to make sure everything's fine...before shipping her across the country, make sure there isn't anything the Linn & the wonderful team would still like to fix in Strasburg...and give those of us in the East a chance to see her!