It is pulled by a tractor engine or by teams of horses.

它是由一辆拖拉机或几匹马牵引的。

And they rolled over on their backs, and they started purring so loudly that it was like tractor engines.

它们滚啊滚，接着开始发出呼噜声，那声音大到像拖拉机的引擎声。