We investigate the use of relaxed decision diagrams for obtaining lower bounds on multi-machine scheduling problems. The type of scheduling problem we consider models a railway service site where maintenance jobs are performed, but is sufficiently general to potentially have wider applicability. We find that our decision diagram formulation is able to give decent lower bounds in short time. Our implementation is competitive with lower bounds given by existing solvers, and is superior when due times are small and have small variance. Further, we find that our model greatly improves upon a naive extension of the state of the art for single-machine scheduling [23].