Greg,
Sounds like a great trip, and a wonderful way to see the USA!

I'll try to answer your questions as best as I can, and hopefully others with experience in these matters will chime in as well.
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1. How easy/hard is it to get from San Francisco International airport to Emeryville station? Seems likes it’s a BART ride then a bus or taxi.
Amtrak has a system of connecting buses called Amtrak Thruway. They are most prevalent in California. There is a bus connecting from various points in San Francisco to the Amtrak station in Emeryville to meet each and every train (including the California Zephyr, Coast Starlight, and Capitol Corridor).
The best way to go from SFO Airport to Emeryville would be to take BART from the airport to the Embarcadero station, which is the last station in San Francisco before BART goes under the bay to Oakland. You would need to walk a long block east on Market Street and cross the Embarcadero (a major street parallel to the Bay) itself to the official Amtrak Thruway bus stop, located adjacent to the Ferry Terminal. Amtrak's bus will then deliver you right to the Emeryville train station in time for your train.
On Amtrak's website, put in the station code "SFC" as the origin, and "SAC" for the destination, and it will show you both the connecting Thruway bus and the Capitol Corridor train you would be taking to Sacramento.
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2. Sacramento. Because of plane arrival time in SF (11am) and the Zephyr departure time, we have to spend at least a night and I thought of taking a train to Sacramento on the afternoon of our plane arrival, and spending a couple of days there. (Seen SF before.)
What is the district around Sacramento’s station like? Is it safe? expensive hotels? (I’m a 3 star hotel man) or to be avoided?
Are taxis available at the station?
Any recommendations for accommodation near the station?
Is the California Railroad Museum worth spending a day at?
As in any major city, be on your guard at night. If possible, find a hotel that has its own free shuttle to pick you up at the station, and verify in advance that they can do this at the hour you plan on arriving. See
here for a list of hotels in the Sacramento area that are either walkable from Amtrak or a light rail ride away. Keep in mind however that if you arrive very late at night, the light rail may not be running at that hour. The light rail, run by
Sacramento Regional Transit, stops right by the station.
Taxis are available in front of the station as well, and they may be a safer alternative for you late at night.
I have not been to the California Railroad Museum, but it has been highly praised by those who have been there. If you have the time and love trains, by all means it's a great way to spend the day. It is very close to the Amtrak station (just walk underneath a freeway overpass). It's also part of Old Town Sacramento, itself worth a visit.
And I think that addresses all your Sacramento questions!

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3. Will it be daylight crossing the Rockies into Denver in September? Arrival in Denver is 6:30pm.
If your train is on time or close to schedule, then yes in September you should arrive into Denver in daylight. And in my opinion, the approach into Denver is the best part of the trip. The tracks wind back and forth as you descend from the mountains to the mile high plain where Denver is located. So you will alternately see the city on your right, then left, then right, etc.... until you get close to the city.
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4. I want to visit the Cass Railroad and the Durbin Rocket in WV, so plan on spending a few days In Washington DC and driving to WV. The question is, what is the best scenic route from Chicago to Washington? The Cardinal or Capitol Ltd ? They both seem to connect with the Zephyr.
I haven't ridden those tourist railroads, but maybe somebody else here has.
The Cardinal and Capitol Limited each offer their pros and cons, and they traverse very different routes. The Cardinal (for now) only runs three days a week, so you will have to plan your entire USA stay based on the days the Cardinal runs if you really want to take that train. It runs via Indianapolis, Cincinnati, Kentucky, West Virginia, and Virginia, finally approaching Washington, DC from the southwest over the same route the Crescent (from Atlanta & New Orleans) takes. The highlight of this trip is the New River Gorge in West Virginia.
The Capitol Limited starts out running closer to the Great Lakes, going through Toledo, Cleveland, Pittsburgh, and then through Maryland, a small part of West Virginia, and back into Maryland as it approaches Washington from the northwest. It runs every day of the week, so its schedule makes the rest of your journey more flexible. Highlight is the morning travel along the Allegheny and Youghogheny Rivers after Pittsburgh, and going through the mountain ranges in Maryland.
The schedule, as of now, has the Cardinal leaving Chicago the earliest, so it may not be recommended if you are making a same-day transfer from the California Zephyr. The Capitol Limited leaves a little later, which gives you more leeway in the event of a late arrival.
More information on hotels near trains in the cities you plan to visit:
San FranciscoDenverChicago (in case of a misconnect or you wish to break up the trip)
Washington, DC/BaltimoreDo enjoy your trip, and if you have any other questions please ask away! We look forward to another great trip report!