Best way to clean plastic passenger cars

Maybe lemon pledge.

I use Windex (blue) ammoniated window cleaner and a soft paper towel on modern MTH cars. Wipe on - wipe off.

MELGAR

I use warm water and soap, along with cotton swabs and toothpicks for an initial clean up before moving on to anything stronger and, usually, that's all it takes. I have also used Pledge to add a little luster, if necessary after cleaning.

Wet wipes...

Don't use WET WIPES!  Many of those have alcohol, that will remove paint and decals/numbers.

If you need a solvent, Naphtha (lighter fluid) is far less likely to damage anything, but it removes all sorts of goo.  As always, test on a hidden spot before using any solvent or cleaner on any finished train item.

Use a paint brush and rob one of your girls large cosmetic brushes.

My wife uses a solution of Borax and warm water to clean delicate figurines.  Never hurt the paint. If you can the cars part and just insert shell in the solution it should work.  

I use a drop or two of Sunlight dish soap, with warm water and a very soft 2”wide paint brush.

  Get s into all the nooks and crannies pretty good

  Then I rinse under warm water and blow dry

 Pledge is a good all around finishing touch 

Al

  Amonia isn't really good for plastics; drawing essential oils from it. Light dish soap and water, Naptha, and Pledge sparingly are my "go too" cleaners. Each one leaving a bit more sheen than the former staying off ink with anything but soap and water.

Good thread, on the adding gloss part, not sure I have pledge. Has anyone tried armor all?

Warm soapy water.   I personally use a very light dawn solution.   The thought of armour all on trains makes me cringe.   That stuff is so greasy.   As far as Increasing the shine you could always re clear them.   

Like other have stated, warm water and a little Dawn does the trick.