Did Mormons attack wagon trains to increase population?

No. The Mormon pioneers never attacked a wagon train to increase their population. They already had several wagon trains of converts moving in each year and could barely build fast enough to house them! There is only one recorded instance of an attack on a wagon train by Mormons, which occurred at Mountain Meadows, Utah. This attack happened during war time and nearly everyone in that wagon train was either killed or run out of town.

Other than that, the dealings of Mormons with wagon trains passing through Utah were positive experiences. The Mormons welcomed wagon trains, because those in the traveling groups were often desperate to trade furniture, clothing, and tools from the east for food and other supplies. It was a win-win situation welcomed by both parties.