Sold Out Frontier ~Reifu-tachi no Kihin Shakou Ressha~

Local nobleman Selena excavates a huge vehicle from ancient ruins. She turned it into a train, built a small town in it and laid tracks for it to move. The established club "Renaissance" inside it becomes a gathering place for all nobles, but underneath presents a prostitution slave camp. Main character is put in charge of training slaves for sex.

[From vndbreview]