Uganda has huge potential electric power from the fall in the water flowing out of Lake Victoria into the Nile. There are a series of falls in that river each of which could be dammed and harvested. But even the first dam, at the Lake itself, built in colonial times, produces huge amounts of electricity, some of which is sold to Kenya. Uganda will not need to buy oil products for the trains, or burn any of the oil from its own small oil field. Presumably the locomotives will change at the border - until Kenya electrifies too.
I have stood on the dam at Jinja and passed it several times on the road and even seen it from the train.
As to why they "opted" for a standard gauge railway, I presume they said 'yes' to the Chinese company that is building it. In any case they need it because the Kenyan metre gauge link will cease fairly soon, if indeed it is still functioning. One big question will be to see whether there will be any actual trade at the western end. Rwanda is developing but will they have much trade? Will the line reach into Congo itself? Would there be any trade there? I would doubt there will be any passenger service beyond Kampala.
Another question of course is what about the metre gauge line to Pakwach and Arua. There was a great deal of publicity - made by Bob Astles - in 1965 when it opened but has it ever generated much trade? If a standard link could be built towards South Sudan - if that country ever ceases civil war - that route might be revived. But otherwise I assume it will cease, leaving Gulu and Lira without a connection. But I doubt if either town generates much trade. Possibly a South Sudan link could follow a new route via Fort Portal, roughly paralleling Lake (?Albert is it still). That would be a shorter route to lay than the old Metre gauge route, but probably not much Uganda trade. And would there be any trade to South Sudan and that corner of the Congo?
I used to think about taking a train to the north of Uganda but never got round to it. The trains were only third class which was not very attractive. I did travel to the north by car mainly to see the National Park and the waterfall.