Well, 18V won't really damage the board, but there is a limit where it is too much. Note that the 18V caution applies to the sound trigger inputs. This really comes down to the power rating of the resistors that limit current to the opto-isolators.
The limiting factor for power input voltage is really the input voltage rating of the switching power supply module, 28 volts. So, that comes in at 20 volts AC on the track. If you look in any Lionel locomotive manual, they say 19V, a similar rating. If you're really worried about the maximum voltage, a few diode pairs will drop the input voltage. 
FWIW, the same basic power supply circuit with the same switching power supply module has been used in my Super-Chuffer for years, and I've never had one of those go from over-voltage out of hundreds shipped and used.
The "choke" or "inductor" mentioned is to protect against degradation of the MTH DCS signal if you use this board in track powered rolling stock.
