Is the DCS track signal bipolar?

There is a negative and a positive rail.

You may find the following interesting. From page 59 of The DCS Companion 3rd Edition:

Home Run/Star Wiring Methods 

While the 2-way communication between the DCS Remote and the TIU is done wirelessly, DCS provides 2-way communication between the TIU and DCS engines through the rails. Although DCS has been proven to work very well using a number of different wiring schemes, it works best when there is a clear path for command and acknowledgement between the TIU and DCS engines. 

This clear path is best obtained when the wires for the center and outside rails are run as pairs. DCS uses a transmission method known as differential signaling to provide improved data communications through the rails between the TIU and DCS engines. Differential signaling transmits information electrically with two complementary signals sent on two separate wires. In the case of DCS, these wires are the Hot and Common wires between the TIU and the tracks. These two wires provide "mirroring" of data packets which serves to make commands more easily identified by DCS engines. 

While DCS engines primarily receive commands through the center rail and respond to, or acknowledge, commands using the outside rails, in both instances the other rails not used provide the mirroring effect of differential signaling. To take best advantage of differential signaling, whenever possible home run or star wiring should be implemented as described below using transformers, TIU channels, and terminal blocks. A terminal block is any device that will one accept of pair of wires as input and then splits that pair into several pairs of wires for output. It's a good idea to use only one terminal block for each TIU channel. 

This and a whole lot more is all in “The DCS Companion 3rd Edition!" This book is available from many fine OGR advertisers and forum sponsors, or as an eBook or a printed book at OGR’s web store!