The photo/sketch is not complete and probably not correct.
1. For DC operation... With the + polarity as marked for Forward movement It appears as though the 2 White LEDs at the Front are connected in series (starting with the anode) and also in series with the 2 Red LEDs in the rear, and there is a resistor R1 to limit the current. [Current flow from + to - ] But with the polarity reversed the 2 White LEDs in the rear (again starting with the anode) are also in series and connected to the 2 Red LEDs in the front, but there is no current limiting resistor. That's not going to work. There must be a mistake in your diagram.
It would work with the existing R1 if the rear white LED connection to the (- marked) terminal was connected to the R1 & Red LED at the (-) marked terminal in your sketch, This would place R1 in series with either polarity.
2. But with a DCC the polarity doesn't reverse. It will just turn one 'side' on or off. So the connected polsarity of the LEDs needs to be corrected/adjusted.
To use only one resistor it needs to be connected to the (+) or common (blue wire) of the decoder... [not a negative lead as shown]. Then disconnect the wire from the rear White LEDs to the (-) marked terminal and connect it to the resistor/Read Red LED side of that connection. [This should be the anode end of the white LEDs]. And disconnect the Red/+ wire of the Front LED string and connect it directly to the REAR light control of the decoder (prob. Yellow).
3. Then reverse the (external) connections to the Rear Red LEDs, and those of the Front White LEDs from that shown in your diagram to your square marked terminals. Then connect the (+) marked wire in your diagram now going only to the cathode side of the Front White LEDs to the FWD decoder lighting output (prob white wire).
Test make sure that it all works with the FWD and REV lighting outputs separately engaged. [If they are both on at the same time it may be too dim due to using the same current limiting resistor.] You may fine that they are too bright. If so, increase the Ohms value of the resistor.
For independent intensity control of Red and White lights it will take some re-wiring and separate additional resistors [but not impossible.]
. OK, so far so good