The secondary costs of running a diesel fleet are also huge! Diesel motors generate a substantial amount of fine particulate that is unhealthy for anybody, and particularly unhealthy for children. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC121970/ Even if an electric fleet is the same cost, changing over would save tons of money and alleviate tons of suffering. It's the right thing to do for any ethical reason you…
It may be more complex: electricity generation is very dirty in some regions, such as where coal is burned.
Is it more dirty than diesel emissions? I have no idea, but old diesels were nasty.