Outside peak hours, urban train companies usually have to park a considerable amount of their rolling stock. In many urban rail networks, such parking takes also place on sidings outside any depot. Obviously, if according to the vehicle schedule a siding is occupied by a parked train, it is no longer available for disposition purposes. In particular, in the event of some unplanned track blockage, depending on the track topology, if some siding constitutes the only option to change from one track to the opposite track in order to continue as return trip, then such an available siding is of high operational value. We propose a procedure to assess significant parts of the operational value of an available siding. In particular, for each siding, we identify an amount of scheduled empty kilometers to park trains in stations that are farther away, that are likely to pay off during operations. This procedure is already in daily use in the planning department of the Berlin fast train company S-Bahn Berlin GmbH.