I have a valid US visa and I will be travelling from Mumbai to US via Frankfurt and Zurich. Would I need a transit visa for either?
The rules are explained in Do I need a visa to transit (or layover) in the Schengen area?
If you have a flight or train between Zurich and Frankfurt, you will need to enter the Schengen area. Depending on your citizenship this could mean getting a full Schengen visa (certainly if you are an Indian citizen).
If you have only one layover in the Schengen area, e.g. in Zurich on the way to the US and in Frankfurt on the way back or vice versa, you probably don't need a visa. In any case, if you have a valid US visa you never need an airport transit visa.