As research has made me painfully aware, 1122-123 lanterns with the large red and green lenses are pretty much unobtanium these days. However, it is possible to make fairly credible looking replicas with common items you find around the house (if your house is a fully stocked parts depot).
Since I don’t, alas, have such a Xanadu, I had to make do with an order from The Train Tender (http://www.ttender.com).
For each lantern assembly, you will require:
1 ea) 1122-177 lantern assy reproduction
2 ea) 711-52 red lens dish type MPC
2 ea) 711-53 green lens dish type MPC
Adhesive of your choice (I used a gel type ACC)
Assembly is straightforward. I tried assembling them as-is and after removing the red and green coloring from the lantern assemblies (helpful hint: Williams’ Lectric Shave is great for removing permanent market and other clear inks/dyes from plastic surfaces). If you leave the coloring on, the light color is richer but a bit dimmer.
Total parts cost per lantern: $2.50. Worth the effort, I think, as the updated lanterns give a better visual indication, particularly at a distance.
Very trolley yours,
-MMM-
A reasonable request, so I hied off to the layout for this happy snap:
And with flash so’s you can see the details:
Here’s the upgraded switch lanterns on display atop a Lionel 1122 switch on my layout. L-R: Unmodded, original lens, MPC lens, one actually in the switch. PCCs L-R: SEPTA, NJPS, PRCo, with a SFMUNI car peeking out from behind the barn (Car! Barn! Carbarn! I got a million of 'em, folks!)
Enjoy!
Mitch
Very nice. I’ll have to look over my switches & see which look I prefer.
Rob
I don’t have any switches I want to change … but that williams electric shave hint was great ! My grand daughter gets into my permanant markers sometimes and has a field day … [banghead]
Heh. Believe it or not, that’s a repair technique where I work. See, I repair hand-held barcode scanners for a living, and when they come back in covered with marker, the Lectric Shave (we actually use the generic) generally dissolves the marker without hurting the plastic! 
Mitch
If it does that … which it does … and I’ve tried everything that did’nt work [:o)] what does it do to your face … [xx(] ? yikes ! Going to walmart and get a bottle …
Well, lemme put it this way: At work, we’re required to wear rubber gloves and eye protection when using the stuff. I wouldn’t put it on MY face… 
Mitch