Welcome to t Gauge and to the Talking T Gauge forum.......
To answer you question......seeing as there's a commercial USB to PWM cable available then someone does not anticipate the power demand on the supply to be greater than 0.5A at 5v. (The spec for USB 2 has a max current specified at 0.5A, and 0.9A for USB3 ). And from my own experience, a power car motor consumes very little current. I've had a set of rechargeable batteries last several hour powering three power cars in one stint.
So....I would think that a power supply that is definitely 5v DC (not AC!!) with a current rating of 1.5A will be perfectly suited to powering a PWM.
And regarding your proposed layout....you are hereby instructed to start a thread in the Layout section giving full details of your plans.