[shmulkey18]

Slave labor was too expensive to use because you risked losing your valuable capital. Instead new immigrants were used such as Irish (East Coast) or Chinese (West Coast).

[abraham]

That's the "slave" labor.

[planetguy]

Slave labour was used sometimes, e.g. the Thailand-Burma railway.

http://en.wikipedia.org/wiki/Burma_Railway

Five hundred men died for every mile of track laid, but hey, they had too many prisoners anyway!