I'd like to use MTH Marker/Class light LEDs in a Lionel TMCC engine and plan to use the +5v DC output from pin 19/20 on the R2LC. I believe that grounding the LEDs to the chassis will damage the R2LC - can someone tell me where I find a safe "-/common" connection?
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I'm not sure why you believe that's a problem, the chassis is the common for those signals. On my diagrams for the R2LC, I don't see any DC ground.
If you're worried about overloading the 5V (maybe you should be, it's pretty low current), just use a diode and resistor and power them from track power. You'll need a dropping resistor anyway, so the 10 cent diode removes all doubt as to the power source.
That is not a bad question to ask since MTH doesn't use chassis ground and that would damage the board, and I think Lionel has moved to some newer electronics that use a DC to DC signal.
But John is correct for the TMCC engines the Chassis ground is common. You won't have any problems powering some marker lights from the DC signal. The 5VDC power supply is strong enough, and some models used that source for Markers.
I've never used the 5V for markers, so I wasn't sure what demands are placed on it. It seems simple enough to add a diode and power from track voltage, all things being equal. 
I don't know about Legacy stuff as far as DC ground, though the R4LC doesn't seem to have a separate DC ground that I can find.
You're right about MTH, they are a totally different beast, it's been interesting working on them recently since I started acquiring them and doing some conversions.
On the earlier LCRUs and the LCRX, the 4 pin railsounds connector pin 3 was a direct output from the Voltage regualtor and normally not used. RS only used 3 wires on the 4 pin connector. So applications like the Crewtalk Caboose used that 3rd pin (5VDC) to drive the caboose side laterns with series LED markers. It will be 5vdc and a nice connection point. G
The 5v on the LCRU was more robust (TO-220 reg); the R2LC and R4LC have a TO-92 regulator. If you consider drawing 5v from this source (not recommended), limit the current draw to 10MA. If you do not limit the draw, you will overheat the 5v reg. John's suggestion to use track power is the best approach.
I knew it was just a TO-92, but I didn't have any idea what demands are already on that 5V supply.
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