Diesel fuel can be cheaper than mains electricity, and has been on a few days when spot electricity prices reached record levels.

I would not expect that grid electricity would REMAIN more expensive than diesel fuel for any extended time.

Also the equipment, whether a diesel generating set or a locomotive costs money to buy and more money to maintain.

When the equipment already exists the position is clearly more favourable. For example the spending of significant capital on diesel generating plant for use a few days a year at times of high grid electricity prices is most unlikely to be viable.

If however the capital has already been sunk on diesel generating plant for say standby purposes, then use for a few dozen hours a year might be worthwhile. Maintenance and depreciation wont be significantly increased by such limited use, which also turns over fuel stocks and reduces risk of it spoiling.

Diesel fuel can be cheaper than mains electricity, and has been on a few days when spot electricity prices reached record levels.

I would not expect that grid electricity would REMAIN more expensive than diesel fuel for any extended time.

Also the equipment, whether a diesel generating set or a locomotive costs money to buy and more money to maintain.

When the equipment already exists the position is clearly more favourable. For example the spending of significant capital on diesel generating plant for use a few days a year at times of high grid electricity prices is most unlikely to be viable.

If however the capital has already been sunk on diesel generating plant for say standby purposes, then use for a few dozen hours a year might be worthwhile. Maintenance and depreciation wont be significantly increased by such limited use, which also turns over fuel stocks and reduces risk of it spoiling.