Geez, disconnect the trains so you can hit both lines at the same time, obviously.

"smallest"?

I reject the premise since there will only ever exist a finite number of people. They will all die. One day the last human will die.

Can I group the people into groups of 1, then 2, then 3 and so forth? When the trolley is done with the killing, it will have killed -1/12 people.

I would pull the lever and then steal a bus and knock the trolley off of the track, killing the least amount of people possible.

sort them so you kill one person first, then 2, then 3, then 4...

Actually... this means there are infinite people so:

Let X be the number of people killed = (-infinity)

As infity is defined :

infinity + X = infinity

infinity + (-infinity) =

infinity - infinity = infinty

So no people would have died black guy pointing at his head meme

there are more real numbers than integers though

You've misunderstood "some infinities are bigger than others." Both of these infinities are the same size. You can show this since each person on the bottom track can be assigned a person from the top track at 1 to 1 ratio. An example of infinities that are different sizes are all whole numbers and all decimal numbers. You cannot assign a whole number to every decimal number.

Matt parker does a good video on this. I can't remember the exact title but if you search "is infinite $20 notes worth more than infinite $1 notes" you should find it.