you obviously misspent your time in one of the un-motorized by-carriages of the S-Bahn, thus completely missing the exiting sound of 800V direct current motors which are especially typical for older (Stadtbahn-)trains. Only once towards the end one briefly hears an E481 (new and on electronic motor steering) through an open door. Too bad!
This is wonderful as it is, as you say, a rare thing to witness never mind be able to record. While it is great to have the complete event here I think you should edit this down into some smaller sections as it is a huge file which will deter a lot of people from downloading. Great upload though!