ORIGINAL: Lieste
Actually worst case for a 'break' is not a few tonnes of gravel, but several thousand tonnes of derailed rolling stock, or a dropped span of a bridge. Neither is easy to fix - moving large numbers of damaged or destroyed wagons and locomotives will require many men or heavy equipment. Bridge replacement can take days/weeks once materials are on hand.
Granted derailing locomotive and associated wagons is a more serious event. But RR cranes actually deal with such disasters routinely -- and they are mounted on rail cars so could be relied upon to approach the given disaster from either side of the problem via undamaged tracks. A RR crane is designed specifically to pick the sorts of loads associated with derailed locomotives or associated rolling stock. The nature of the repair or track clearing isn't something that delays strategic movement over the course of weeks. The Pick of even a locomotive occurs over the period of hours not weeks. And as we are talking about a single rail line that is of the utmost strategic importance in terms of supplying an entire Army Group, such equipment would be readily available in order to deal with derailment issues quickly and restablish movement along this ONE very important track.
Regarding Bridges, while I am not aware of any track that span rivers falling victim to the personal hell [:)] I am experiencing with the new WiTE partisan warfare, the only way to effectively put a long term hit on this sort of robust structure in reality is to destroy the abutments and bent supports. Both of which would typically be relying upon an extensive foundation system and huge concrete pile caps due to the loads being carried by the bridge spans. Neither of these elements is "easily" destroyed by a few pounds of explosive. We’re talking hundreds of pounds of explosive to be able to make dents in the like. I think the logistics associated with such a demolition would likely be beyond the means of a typical partisan band. That is unless they are operating out of swamps and forests with 18-wheelers packed with dynamite. Military bridge demolition is typically focused on the spans rather than the bent foundations and abutments. They are easy targets for explosives. But by the same token, the actual spans can be quickly repaired. Again the rather heavy materials required for the repair are easily transported almost right to the point of the demolition by use of the undamaged tracks on either side of the damaged bridge. This to include either heavy steel I-beam\trusses or heavy timber elements for reconstruction of damaged bridge trusses. Trusses or spanning elements were often prefabricated sections which could be puzzled together on the work site quickly. Certainly these sorts of repairs could be completed and functioning in terms of movement points expended within a given turn rather than complete loss of rail roads or RR bridges that require multiple game turns to put back into operation.
But having rambled on about this, I would be interested in historical examples of Partisan bands\gaggles completely obliterating heavy steel or concrete RR bridge structures and the associated time required for repair of the like.
Conversely repair times required by combat engineers\pioneers of bridges (both road and rail bridges) that were destroyed by opposing combat engineers is available. Read into the above, the level of destruction of a strategically important RR bridge by a band of free wheeling Partisans is unlikely to be of the same order of magnitude as what would be accomplished by combat engineers or pioneers when tasked with the same.
Sorry, but being a civil engineer by profession and having actually designed RR bedding and bridge foundations for a variety of different subgrade conditions, perhaps I have more faith in the ability of combat engineers to rapidly deal with these sorts of issues.