stalling of locomotive on Atlas Switches

I think the short answer is 'no' you can't easily add a capacitor to prevent stalling.  I asked a similar question once, and the response was that the capacitor would have to be very large, in some cases the size of a small soup can.  To get more detailed replies you'll have to be specific.  Which "older locomotives?" And which year/model/style of Atlas switches??

A lot of MTH steam locos made in the '90s came from Korea with only two pickup rollers, frankly not enough for complex track arrangements.  Despite having a (small) flywheel, the self-locking gears don't coast worth a darn.  Combine that with slow running over certain switches and you're likely to have frustration.  Gunrunnerjohn and other MTH techs have done custom jobs for folks, adding a roller or two to the tender trucks.  You can probably contact one of them through the email on their Forum profile.  For a diesel, you could tether it to a dummy unit if your mainline is long enough to warrant always running two units.  Also consider the ground side of the circuit.  Rubber tires can deprive a loco of vital ground pickup.  MTH's "Proto 3-2" feature is another culprit.  Whenever I work on a loco with this feature, I bridge both outside rail feeds together.

Early Atlas switches had some issues which were addressed in subsequent redesigns, however the SKU numbers didn't change.  Supposedly the newer releases say "UV resistant" on the package.  Personally I avoid using curved switches, or any kind of "special" switches (3-way, 4-way, double slip, etc.) in my layout plans.  Check your switches with a multimeter and make sure all of the ground rails are actually grounded.  Unlike Lionel tubular, plastic ties don't automatically tie the outside rails together.  Hope this helps!

I have one steamer with this issue and fixed it by applying hvac aluminum tape to the plastic frog part of the switch. Technically I don’t think it’s the frog but the plastic part that fits in the frog of the switch