If the 4.5V DC for FedEx is derived from your train transformer AC output using an AC-to-DC converter module, then you can't use the shared AC-DC common method. You would need to use a relay. I suggest you "dedicate" one AC-to-DC converter module to the FedEx forklift as shown below. In other words, the FedEx forklift is now powered by 14V AC (or whatever Accessory voltage you use).
Of course this means you no longer can use this AC-to-DC converter to power your 4.5V DC cars or other buildings...unless you want them to turn on/off with the FedEx building!
Add additional AC-to-DC converter modules to power the "always-on" 4.5V DC devices.
I suggest this alternative (vs. a relay) for several reasons which are technically boring and cause eye-glazing. In addition, a suitable AC relay is ~$15 whereas an additional AC-to-DC converter module is closer to $5.