>The current ACELA fleet requires two locomotives per train set because of its great weight due to earlier crash standards compared to the crash energy management approach in newer designs.

Here is where there is a conflict between typical US safety practices and other countries.

In the US, there is a huge focus on crash survival. Other countries focus more on crash prevention. Obviously crashes will happen, it is inevitable. The question is a matter of physical mass, to be survivable, the equipment is typically heavier. The greater the masses, the more crash energy that must be dissipated.

Lighter equipment might not be as survivable in an actual crash, but as long as they don't crash, it takes less horsepower to move the resulting lighter weight. When they do crash, there is a lighter mass involved. Of course, mixed equipment ddoesn't fare well on the lighter equipment, one reason the FRA now insists on physical or temporal separation between heavy rail and light rail operations. Regardless how safe a compact automobile may be, it still won't fare well in a high speed head on or tangling with a semi.

As far as equipment age and replacement - there are far too many factors at play here, but it comes down to a simple matter of value. What is the replacement cost, present and future operating cost (including maintenance and fuel/energy efficiency), maintainability (design and parts availability), reliability and the actual need for the equipment. Why do you think most freight railroads are starting overhaul and upgrade programs in their older GE's now. First, they have a huge fleet of them and are approaching the end of their economical life. Second, from all accounts the Tier 4 replacements are too expensive, too maintenance intensive, unproven and less fuel efficient. So for many railroads, rebuilding older proven locomotive designs is a far better value, especially considering how PSR is downsizing their active fleets. So many of the oddball, expensive to operate/maintain or bad actors will simply be retired.

What I have found on the railroad is that if the mechanical and operating crews like a certain design of locomotive, they tend to take care of them. If they don't like them, they don't take care of them which shortens their operating lives.