Diesel, like the vast majority of trains in the US. Even if the US performed electrification of trains, this would be a low-priority to electrify because it's not a major shipping route.
It does.
> this would be a low-priority to electrify because it’s not a major shipping route.
US rail electrification is more focused on passenger routes than shipping routes; an electric freight engine for mainline use has been introduced (just last year), but its not intended as the sole power source for a train, but rather to improve efficiency by being used in a consist with diesel engines, recharging with regenerative braking, reducing overall fuel use.