Hi. I haven't been to Sacramento in a few months but I live in the Bay Area so a lot of the restrictions in Sacramento are similar or a bit less restrictive. The below are the facts to the best of my knowledge:
The Sacramento region exited the Statewide Regional Mandatory Stay at Home order on January 13th and the Limited Stay at Home Order on January 25th. With that said, they are now in the Purple Tier/Tier 1 which is the "Widespread" COVID Tier and is the most restrictive outside of the Regional Stay at Home Order. The majority of the State just left the Stay at Home Orders and went back "down" to the Purple Tier.
To summarize for Sacramento, restaurants have been allowed to go from take-out only to take-out and outdoor dining with modifications in place. Bars are not allowed to be open unless they having outdoor dining. Most retail and malls are allowed to be open at 25% capacity (grocery stores at 50%) with modifications as long as no food is consumed inside. While museums are allowed to open again outdoors with modifications, The California State Railroad Museum is operated by the California State Park System. I don't recall them being open for the past 8-10 months though the outdoor park area is now allowed to be open.
Here is the Sacramento County Health Order for your reference:
Sacramento County Health Orders.
Martinez is in Contra Costa County which is also the county I live in. We just left the Regional Stay at Home Order on January 25th and are now in the Purple Tier as well. Having been in both areas, Downtown Martinez is near the station and fairly walkable (about the distance from the train platforms in SAC to the station building itself). There are a few dining establishments and a hot dog place that I stop at near the station. Usually met a few others from a volunteer group I am with there on occasion and ate outside at the station or in the large regional park area across the tracks (easy place to meet via Amtrak). I haven't been in Old Town Sac recently so not sure what the options are up there. Martinez is a bit less of a tourist draw than Old Town Sac so the prices may be more reasonable.
I time my connections a bit more closely and am often found running from one train to the other (yes a bit risky timing wise I know!), but taking the Capitol Corridor down and back would be perfectly reasonable for the average person with the time you would have (if on schedule). If you wanted a straight shot you could always ask the Conductor on 5 if you could extend to Martinez and/or Board 14 in Martinez (coach extension MTZ to SAC or what not). The Capitol Corridor would be a nice Round Trip as well and you could always buy the ticket last minute depending on how 5 is doing on time.
Keep in mind the schedule is greatly reduced from the norm, however there still are quite a few train options:
Capitol Corridor Schedule.
As for the train station in Sacramento, it has undergone some work since 2014 and is still undergoing work. The Amtrak ticketing area and baggage claim have also been "improved" along with their staff office space and crew base. The only station within the region that is actually owned by Amtrak is Emeryville so a lot of this project has been part of a larger Sacramento Rail Yards District improvement project. More work is to come but it hopefully will turn out quite nicely.
Do note that in California, a Mask is required anytime anyone is within six feet of another person inside AND outside with few exceptions (medical) and if walking outside, a mask must be readily available to put on should you come within six feet of anyone else not in your immediate household/living group. Also at restaurants, you must be actively eating or drinking to take the mask off (i.e. mask on when waiting to order or sitting at table without food/drink etc...). This is the norm I am used to so I have no idea how that compares to other parts of the county. I have a trip in February taking Amtrak (essential travel for work) to Chicago, New York, Philadelphia, D.C., Orlando, and Tampa so I will soon find out. A lot of research in the mean time of the various travel rules and restrictions.
The Sierras (and most of NorCal) are taking a beating with the storms right now so it should be a very scenic ride through the mountains (not that the Midwest is a stranger to snow!) I hope you enjoy the trip!