Need help with MTH Street Lamp Posts

MTH made many different versions of these; some came pre-wired, or with old-fashioned binding posts on the base or clips for the wires underneath the base. I think that the first thing you have to do is determine whether the bulbs in them are incandescent or LEDs although I believe most made in the past were the former although MTH also made an LED set. If incandescent they will work on AC transformer power and the polarity of the wires won’t matter. You could try hooking one up to transformer accessory power at say 12-14 volts and see if it illuminates. Some model train incandescent bulbs are rated at up to 18 volts but will burn out more quickly if kept at that level.

When I still used these I never tried wiring them in series but instead hooked them all up to transformer accessory power individually or in parallel. 

If they are LEDs you need a DC/rectified power supply and correct polarity must be applied. The MTH LED set came with the wires already attached and color-coded for + and - but I don’t know whether they came equipped with resistors to rectify transformer power. Series wiring is preferable here but I have only done this with passenger car interior lighting using a power distribution board that already has the wiring points printed on it. This is a fiddly job and unless you actually have LEDs in these lamps it is best avoided. OGF sponsor EvanDesigns makes DC power supplies for multiple LEDs and you might check their website if you need to go down this route.

Don't know specifically about MTH lights, but in general incandescent "AC" lights will work fine on DC of the appropriate voltage, regardless of polarity, and LED "DC" lights will work fine on AC, as long as there's a current limiting resistor in the circuit somewhere (the "D" in "LED", after all, stands for "diode"). As to DC polarity, the only effect of reversing the DC polarity is that the light will not work as long as the polarity is backwards.

So, to test the lights, you might want to use the throttle-controlled output of one of your transformers, start at the low end, and increase the output until you get the desired brightness. Then, measure the voltage and recreate that level on the feed running (in parallel) to the lights you want to use.