lionel 0 48 command switch controller doesn't light up

Did you change the connections to add a longer cable, or in any way??  If so- you likely did not re-connect the wires 100%- it's easy to miss.  If you have not gotten "under the hood"- and something has gone wrong, then I understand the LEDs are not user-servicable.   

If it is under warranty (1 year) simple- call Lionel Service.

If not, You could swap the controller unit with one you know works, and see if it works on the 048 switch.  Or- you could send it to Lionel and hope they just fix it or exchange it- you have nothing to lose except the cost of the shipping.  BUT- they have to agree to accept it- so call Service first.  If they plan to charge you they should call with the charges beforehand, and you can have them send it back as is, or pay to fix it.

I have the same issue with one or two- and I just decided to live with it.

Also- you didn't say did it light properly before?? If it did and now it doesn't,  AND you haven't changed anything, then the Lionel Service might be the best option.  It could be that their cost to fix it is more expensive than a new controller (on e-Bay) though.

https://www.ebay.com/itm/Lione...8:g:BWcAAOSwmtJXVynt

The controller uses all LEDs. They are replaceable if you are competent soldering and matching the type of LED.

As you can see, there is a diode in series with the green which may be open or bad solder or trace on the board. Since you have red, then I would think your wiring is sound. There is also a chance that the signal on the yellow wire which goes from +5 vdc for green to -5 vdc for red is faulty, missing the +5 vdc. If you have another switch controller, swapping them will tell you if that is the problem.