Unrailed!

If you're just going for this specific achievement (or want to get it out of your way), there is a way to speedrun it. The only requirement is to play endless long enough to unlock dynamite wagon and its upgrades (3 to be safe).

Pre-game settings:
Go into Start Game -> Local
Seed: Animaig -> This will automatically set it to Sandbox, Medium.
Buy one upgraded dynamite wagon (I used the third upgrade), place it as first wagon after the train. Buy the tracks wagon too, although you won't use it.
Start it as soon as ready.

As you can see, there are two ducks directly above the starting station. Immediately go left and grab your wood. Move to the 'light green' tile and build a 3 tiles long bridge towards the bolt. Go back, grab a dynamite and place it at the end of the bridge, this will kill both ducks. Give it few seconds (like 3-4), press start on your controller and restart. Move towards start game again. The whole loop should take around 23 seconds so you should be done with this achievement within 20 minutes. Crash the train naturally (just don't place tracks) to ensure you properly 'finish' the game.

[UPDATE] This achievement now unlocks. It popped retroactively for me and should unlock as normal now.

This achievement is currently unobtainable

As of February 2021 this achievement cannot be unlocked. I've killed more than 100 ducks with dynamite and tried every fix in the book to get it to pop. This achievement has never been unlocked by anyone.

[UPDATE] The devs know about the issue and have said that it will be fixed in the next 1 to 2 updates. However, no timeline or dates were given.

Solution

To unlock this achievement you need to kill/explode 100 ducks with dynamite that can be gained from the dynamite wagon. You can only find ducks in the first biome (grass/plains). I would wait to go for this until after playing some of the game, as I was at 15% just through natural play.

Use the following settings for the easiest grind:

Mode: Sandbox
Difficult: Easy
Engine: The first one
Wagons: The 3 required wagons and then at least one dynamite wagon, but I would suggest two.

Play through the game normally, making sure to keep your train alive and getting it to the other side of the map. As you're going through make sure to kill any and all ducks you see with sticks of dynamite.

If playing co-op, get one person to focus on making the track, while the other to focuses on mining resources and blowing up ducks. If you're playing solo, just set the bot to constantly make tracks, this will allow you to only have to focus on mining resources and placing the tracks (as well as blowing up ducks)

As you go through the stages you can also grind this achievement out at the same time by purposely dodging the train stations.