In the sentence “Le train est en retard à cause de travaux sur la voie.”, should one insert “des”, “les” or nothing between “de” and “travaux”?
In this case you can either say à cause de travaux or à cause des travaux (which is more or less the contracted form of à cause de les travaux which is not used in French. This is grammatically incorrect.). The first expression means that you don't know which construction work or maintenance operation(s) are realised on the railway track but you do know that there is something. The second one means that you already mentioned the work on the railway: it is something known by you and the people whom you are speaking to.
The first expression uses the indefinite form of the plural of travail while the second one uses the definite plural form of it (the difference is the same as between one school and the school in English).