>>2040391
The most extreme example I can think of is Strasburg in Pennsylvania. They've basically been a steam heritage railway since the late 1960s, but around 2010 local businesses contacted them about running revenue freight. After studying it, the board agreed, a couple see of older bridges were replaced with modern concrete viaduct and Strasburg started running revenue in 2013. Which means if it's an excursion day and the steam locomotives are steamed up, they make an early morning trip out to exchange cars from the NS interchange and you can see 100 year old steam hauling modern 100 ton frieght cars in revenue service.