This is a relatively short mission - thankfully - so head back to the 'L' on the map and ride with Luisa on the carriage to the bridge where you’ll plant the explosive ambush for the impending convoy.
To wipe out the vast majority of the convoy in as short a time as possible, you’ll want to spread the placement of dynamite properly. The convoy will come from your left (if you’re facing the track from the detonator), so focus your line from that direction.
Your last stick should be left close to the bridge, and then run up the hill towards Luisa and the detonator itself. The charges won’t all blow at once, so be sure you’re happy with the controls detailed on the screen.
You need to 'prime' the charges first before blowing them, and the real trick here isn’t to detonate the one on the left first. You want to prime the one closest to the bridge first, so when you do blow it, the majority of the convoy will already be over the other sticks of dynamite.
This way, once you’ve blown the right-most dynamite first, you can quickly prime and blow the remaining charges from right-left and take more of the convoy out! This leaves you mopping up the remaining stragglers with your rifle from where you are, high up on the hill. Mission complete!