I am not going but spend as much time as you can in Saint Petersburg.
The first 20 or pages of my travel narrative are devoted to this city.
You can get it for free here:
Also, its easy to get to and from Saint Petersburg via Helsinki by train, so I would consider flight options there to see if they are cheaper.
Not sure where you are flying from but via NYC area, r/t fares to Moscow are typically under $500 USD.
I'm not going, but I've traveled around Russia a good bit.
The World Cup would be a great time to travel beyond the typical foreign tourist targets of Moscow & Petersburg. All the cities are easy to reach by train, but this event will make them extra visitor friendly. The other World Cup sites I'd particularly recommend are Nizhny Novgorod & Yekaterinburg. Just 2 days in each/either city would give you exposure to a very different part of Russia.
You can book trains easily & in English through the official site, no need to use any service that charges a big markup: https://pass.rzd.ru/main-pass/public/en
Let me know which American Credit Card works for you...when I went to book on their official English site it did not accept ANY of my American issued credit cards (14 or so). Shame becasue the site is so well done in English.
@oliver033 Great point, this was a headache not only for Russian Railways but also domestic Russian flights: they absolutely require cards have Mastercard SecureCode / Verified by Visa ... and hardly any US issuers offer these, because they didn't catch on here. (For example, I believe Barclaycard used to offer it but discontinued in 2015.) The one thing in my wallet that worked was a Bank of America debit card; I believe Wells Fargo & PNC credit cards also support these technologies. And if you have an eligible card, it's a feature you need to explicitly activate, and the Russian websites aren't the best on telling you that's the problem.
Also, I'm jealous of the new English site: I last visited just before it rolled out, so I had to navigate the Russian site using a combo of college language studies, Google Translate, and info from the Man in Seat 61 (great resource for train travel worldwide).