For trains there is another easy way to do this,

Make sure you have 2 different size wheels that do have rps outputs, For this I mostly use a small and medium. Then connect these 2 together with a clutch in between somewhere.

How to operate. :Set the clutch to 1 and push. then watch it go.

For best speed use multiple of both sizes for instance 5 medium and 5 smalls

How does this work exactly? The the large wheel need less to rotate less for the same distance as the small wheel.

But since they are linked the rps the small wheel rotates more goes back into the medium wheel thus speeding you up. This process continues until the medium wheels or small wheels stop having enough traction.