Currently, only the loco you are driving is contributing power. However, I have gotten 3 locomotives to pull a coal train up a very very steep grade the other night using the follow me mod.. more to come on that.
As for the GP7, yeah it is weak. the GP7 was one of the very first diesel locos that NA railroads were willing to give up steam power to convert to, however it is very underpowered compared to the large road locos you see now. So I made it proportionally weaker so that it felt real!