Is it normal that the line be fainter than the control?
Its an often occurance. It means nothing. Even synthetic urine controls (which contain no drug metabolites at all) can produce a faint line on an assay.

If youre still worried and unconvinced, you can always go for substitution (with either a clean persons sample or synthetic). Subbing is easy on a DOT test because you wont be observed. Observed collections only occur if the donor had been suspected of tampering, or if the test is a follow-up/return to duty test from a previous postive result (after the donor had completed the SAP process, as required by the DOT regulations).