If only the last mile is done by truck, road maintenance taxes will have virtually zero impact on price.

In Europe, a truck may transport about 250000 packs of yoghurt. (Assuming 100ml packs.)

If the truck pays 3 Euro road maintenance tax per 30-mile trip from and to the railway cargo hub, that comes out at 0.0001 Euros per can of yoghurt, a hundredth of a cent.

3 euros seems low, or maybe the road use tax here is just too high.

One EV road use tax in Australia (Victoria) is 2.5c per km, I'll assume that's fair for now (I doubt it is, but anyway). A prior post said a truck does 5,000 times more damage than a car, which means the truck should pay $125 per km if it's fair. So a 30 mile trip would be roughly $6000. About 2.5c per yoghurt.

Not a lot. But it's on each item and will add up. Not sure how often trucks run empty to and fro either, someone has to pay for that. Petrol excise here was halved to 22c/L and my car uses about 8L/100km so that 2.5c does not seem too unreasonable.

What kind of distance are you talking about in real terms for "the last mile"?

A few hundred miles or so?

Trains

So you have to build railway lines everywhere. What's the ecological impact of all that? Where's all the concrete for all the bridges going to come from?

Everything has an impact. Rail has a lesser environmental impact than roads and cars.