Planning Leskovac

The attack of September 6th was a part of Operation Ratweek.  A description of this operation can be found on the internet.  This description was found on Wikipedia: 

Operation Ratweek was a series of coordinated attacks on the Axis forces' communication lines in the Balkans during World War II. Launched on 1 September 1944. the attack was led by the combined operations units of the Yugoslav Partisans, Land Forces Adriatic, the heavy bombers of the U.S. 15th Air Force and the light and medium bombers of the Balkan Air Force.

The Wikipedia site goes into great detail about the planning of the Leskovac mission:

The main withdrawal (of German Forces) from Greece was expected via the Vardar Valley and the Belgrade-Salonika railway. This was identified as the key target, and at the end of August 1944, Maclean* flew to Bojnik in Serbia. There, he joined the local commander Koča Popović and his BLO John Henniker-Major to organise the destruction of the railway at Leskovac. The three officers agreed on the plan of action and relayed it to the Allied force HQ at Caserta. Maclean was left with the local commander of the 24th Partisan Division to work out the details.[9]

The plan was for the Partisans to attack the two points north and south of Leskovac, while the town, the seat of a large German garrison, would be left to the Allied air force. On 6 September, the day of the proposed attack, an urgent message from William Elliot arrived stating that the air reconnaissance confirmed the presence of a strong concentration of armour and motor transport in town. Soon after, an air force of fifty Flying Fortresses bombed the town heavily in a surprise attack, reducing much of it to rubble with a large number of civilian casualties.[10]

The attack on the remaining railroad went ahead on the same night, with the Partisans blowing up small bridges and culverts as well as tearing up the sleepers and setting them on fire.[11] Strategically, the operation was a success as:

It would be some time before that particular stretch of the Belgrade-Salonika railway was again open to traffic. The enemy forces in Greece, if they were to get out at all, would have to get out by road or by sea, a hazardous proceeding in either event. If everywhere else the Partisans had done their job as thoroughly as here, RATWEEK would have got off to a good start.

— Maclean, p. 489 **

Note that the Wikipedia description says the attack was carried out by B-17's. The actual attack was executed by the 376th.

* Brigadier Fitzroy Maclean, Allied representative at Yugoslav Partisans HQ at Vis, and the commander of the Maclean Mission

** Maclean, Fitzroy (1991). Eastern Approaches. London: Penguin Books. ISBN 978-0-141-04284-8.

The pre-mission planning documents, which I found in the records of the 376th, confirm this background.  Below is a copy of specifics info sent to the 376th.

This is an enlarged image of the text.