R/C battery PS-2/3?

..what is a battery RC..assuming you mean remote control..DCS handheld is an RC unit.

Dead Rails.

Not hard to do.  Just use a 12-20V battery with fuse and on off switch and feed it to the Red and black wires going into the 7 pin connector.  Now the hard part is getting the TIU involved!  For G scale the battery feed the input of a TIU removed from the case that is in another car.  Box car or covered hopper.  The output of the TIU goes to the engine red and black as mentioned above.  G

What is the physical size of the TIU unit to which you are referring?   I am only aware of the large TIU units we have installed around the layout.   

Anyway to not have a dedicated trailing car with PS-2/3? 

I would use the PS effects in conventional R/C battery,  only eliminating track power.

Check out: https://ogrforum.com/...01#79283631153848901

As the whether you can do this without a dedicated trailing car, I don't think so if using PS-2/3 as the control method.  That is, as described in the link there are only 3 devices (TIU, Explorer, DCSRC) that can generate the DCS control signals to the engines.  

In your OP, does "conventional DC plus battery R/C" mean you only need forward and reverse operation?  If so, most PS2 and all PS3 engines can be controlled by conventional DC.  So you would need some remote controllable receiver that takes fixed-voltage DC battery power and generates DC voltage of either polarity (for direction control) and variable voltage (for speed control).  I'm assuming that you want the PS2-3 electronics left in the engine so that you retain some of the PS2-3 features such as sound and smoke that are still functional under conventional DC. 

If you restrict yourself to PS3 engines that have DCC capability, you could use a battery-powered R/C receiver that generates DCC commands.