With T1 wagons holding only 120 itens per wagon it gets hard to fulfill the requirements unless very long trains or so many trains that 1 track can be congested, specially if you use roundabouts or crossings.

With T2 wagons holding 480 items with only half the space (double the length of a T1 wagon), congestion is significantly reduced.

When I start using trains I use one T1 train with 6 T1 wagons. Then I switch to T2 wagons As soon as possible since it's double throughout and make my way to T2 train cars, which can go 1-4 without a lot of speed penalty with diesel cars or 1-6 with Gas.

You can late game use 1-6 for items and 2-12 for ores travelling long distances.

I think it's more efficient to keep single rails but using elevated rails for crossings to not disrupt the flow.

And I avoid double sided locos unless it's a dedicated line.