The #90 is simply a doorbell button (one normally-open contact) with a Lionel trademark. It connects two wires when you press the button. It can be used to activate any accessory that requires a single wire that activates the device, such as the gateman, the switch tower guys, a coal loader, log loader, etc. The switch tower, as an example, uses three wires. There is a common, a constant powered wire for the internal lamp, and a switched wire that activates the motor.
The signal accessories that have three wires, such as the 153, and the 154, have a common lead, and a hot lead to each of the two lamps. You really don't want to eliminate any of the wires. To use a single pushbutton contact for the 153, you would need to use a simple relay that duplicates the action of the 153C contactor: the lead to the green lamp is always on until the train pushes down on the contactor, causing the green lamp lead to open, and the red lamp lead to close. You could also use a pushbutton that already has the open / close contacts built in. I'm not sure Lionel makes one, but they are available cheap at Radio Shack or online.
To use a #90 to activate the 154, you would use the pushbutton to activate a circuit board that is designed to provide alternating pulses to the two output leads, each of which goes to one of the red lamps in the signal. I believe that Dallee and several other manufacturers make them.
