Roadtrain

The Roadtrain is a tractor unit. The rear two axles are powered. The bullbars on the front are very distinctive.

One spawns at the Angel Pine Sawmill in Whetstone, south of Mount Chiliad. They can also be found in the Fort Carson area.

 

More info:

gta.wikia.com/wiki/Roadtrain 

https://www.grandtheftwiki.com/Roadtrain