Information to consider.
- (1.) Automated speed control ??
- (2.) Are your lamps compatible?? Often lamps are upgraded to 18 volts to operate at higher TMCC operation voltage.
- (3.) Do you want electro-couplers?? There are different lengths and mountings.
- (4.) Components most likely are installed in the tender. Size of the tender is a consideration.
- (5.) The tender/loco tether, usually is changed to a 6, 8 or even 10 pin tether to accommodate new and existing features. You would need to know components mounted in the loco and how many wires required to connect to the control board in the tender.
- (6.) Install instructions are available on the ERR website. A huge plus, as has been mentioned. Understanding the install allows, for an accurate material list of all components required.
- (7.) How are you installing a TMCC antenna ?? There are a couple of different ways this is done, usually with an isolated tender shell, which may require special plastic screws or mounting bushings.
That's a start list, best wishes with your project.
Mike CT. 
