There is no need to put any additional command AFAIK.
At least I never had to, in my years of experience with Subways and Denebs.
First of all... and since you don't state... is your Amiga having an accelerator with RAM?
Also make sure you install Poseidon correctly and after a reboot (to give User-Startup the chance to load PsdStackLoader) open Trident from SYS:Prefs and add subwayusb.device under Controller and hit Online.
If everything was done correctly you should have a green light next to the declaration.
Hit SAVE and you're set for good.
