A drive train, especially a vehicle drive train, includes: an engine for supplying drive power into the drive train; a cooling circuit in which a cooling medium is revolved in order to cool the engine or an electric generator or another unit; an expansion machine which is driven with fluid or steam as a working medium and by way of which additional drive power can be supplied to the drive train or which drives an electric generator or another unit, the cooling medium of the cooling circuit being simultaneously the working medium of the expansion machine; and a bypass to the expansion machine which is provided through which the working medium of the expansion machine is forced through a switching valve or can be guided past the expansion machine automatically by the prevailing pressure conditions.