Ray and Jerry -- the link to the eBay converter provided by Ray earlier goes to this label: "JacobsParts Mini360 DC Voltage Step Down Power Converter Buck Module" -- based on both the title and the specs in the listing, this is a buck module only, not a buck-boost module.  So if you are running Legacy or TMCC (?), etc. at a fixed AC track power (usually something like ~18 V), then a buck module should be all that is needed.  So this device would work, as would the device that GRJ devised - which has the added advantage that it does the AC-DC conversion on the same board (widely discussion on other OGR fora).

However, if one is running conventional variable AC, then a buck (only) converter might work if the LED strip lights work at something like 4 VDC, since the track voltage is - for most practical purposes - going to vary between ~4 VAC and ~16-18 VAC.  In this case, a buck converter will operate 'correctly' and simply step the voltage down to a user-set constant output voltage (that is always lower than the supply voltage).  But if one uses the common LED strip lights that are spec'd at 12 VDC ( and perhaps operated around 9 or 10 volts to control the brightness) with variable AC track power as input to the buck converter, there will be times when the supply voltage is less than the output voltage set point for the buck converter; in that case the buck converter output goes to zero (or near that).

A search for buck-boost converters on eBay will yield a lot of (mostly Chinese) buck boost boards that I'm sure would work - though I haven't seen any that are very small, which is one of Jerry's requirement for his coaches.  My assumption is that a lot of the buck and buck - boost boards that operate in the ~1 to ~20 VDC range and seem to be widely available on eBay and Amazon are designed for automotive systems.  I wonder if these are 'over-spec'd' compared to what the requirements are for a board used to control voltage to an LED strip for lighting model railroad passenger cars.  I'm not enough of an electrical engineer to know that - but perhaps there are circuit diagrams, etc. for a buck-boost system that meets the needs of the model railroad community - including small size.

- Rich

ps  Jerry - the car interiors look fantastic -- the couple of close-up shots you posted are indistinguishable from the "real thing".