since St Lazare is a big station, two levels,
St Lazare is one of the smaller stations, certainly smaller than Gare du Nord, Gare de l´Est, Gare de Lyon or Montparnasse. I think that there are actually 4 levels at St Lazare but all of the trains depart from the top level.
Fred, are you sure that the SNCF ticket machines at Gare St.-Lazare do not take US credit cards?
Firstly, your card must have an EVM chip. The ticket machines do not have magnetic strip readers. I should think that by now, all US issued cards have a chip.
Secondly, acceptance will depend upon the processing rules established by your card issuing bank. Occasionally when purchasing a monthly Navigo fare, my US credit card is sometimes refused when using the RATP ticket machines. When this happens, I go to one of the train stations and use an SNCF ticket machine. SNCF machines always work but they also always ask for my PIN.
I shouldn´t think that using an SNCF ticket machine would be a problem with a US credit card as long as your bank has issued you a PIN but not all US credit cards have the same purchase rules.