Unrailed!

Lava blobs are the orange cubes you can encounter in the Hell biome, but they are not always easy to find.

The lava blobs will bounce slowly towards your train to set it on fire or sometimes just sit in the lava(you can build a bridge with iron next to it to get an easy extinguish). You need to pick up your water bucket and go near a lava blob for it to turn grey, this counts as 1 extinguishment and will empty the water bucket. Note that they can and most of the time will reignite a couple of times so you need to fill up the water bucket again and be ready for it. The water bucket can be filled at steam vents found in this biome.

After 3-4 times they will run off screen, never to be seen again, so you need to find another. You can extinguish the same lava blob several times and it will all count towards this achievement!