A second car would knock it down some, but remember it's basically a voltage divider, so when you start with a lower voltage and divide it, you don't have the same absolute change. OTOH, just seeing it go from 15V to 4V with one car suggests that the subway with multiple cars would be a major issue for DCS. 
One question that should be asked here, is the voltage drop we're seeing with your measuring board the same as we'd see with the TIU? I suspect that the TIU may have a lower impedance driving the track than the board with 100 ohms in series with the output.
Adrian, how close does this track to the behavior of the TIU signal?
