We will be landing at SFO with a baby, baby’s stroller, and tons of luggage. Then’ we will be heading towards the car rental center to pick up our rental car. Looks like the Airtrain is the practical way to getting there, but we’ve never done that before.
How convenient will it be using Airtrain for that purpose? Will it be just getting into the train on one end and getting off at the destination? No extra stairs or other obstacles between point A and B?

It's quite easy.

It's a short walk from the terminal to the Airtrain station - there are escalators & elevators to take you up to the platform level.

At the rental car stop you just step out of the Airtrain and you're right at the counters, then you take an elevator down to whatever level your car is on.

You can bring luggage carts on the Airtrain - and simply roll them on/off through the wide doors.

ps
Please place luggage cart brake 2 on. (incessant recording)

Yes, but there is an escalator at the rental car end, IIRC. No doubt an elevator nearby.

Even better would be to leave one person in the terminal with baby and encumbrances while the other person goes to get the car and comes back to pick them up. The person in the terminal will have to be patient and have faith because this may take a non-trivial amount of time (train schedules, lines at pickup, finding the terminal again) but it saves a world of hauling and hurrying to get through doors.

Even better would be the "car person" going to the baggage area with the "baby person" and engaging a redcap to help with the luggage. Once you have a porter, the "car person" leaves, and the porter gathers the luggage and takes it to whatever door for pickup. I assume there are still porters at SFO; they continue at Boston Logan.

Same in reverse when you leave.

Notice I have said "person", not "mom" to remain in the terminal.

I'm not sure I agree with Ackislander about splitting up. In my experience it's generally less hassle as a family to stay together, take your time & use the elevators to get up to the Airtrain, get the rental car, then be on your way at your own pace.

Separating at a busy airport, navigating back to the terminal from the rental car pickup, leaving one person to retrieve bags from baggage claim and handle baby/stroller (or find a porter), then reconnecting curbside is more of a pain.

Ordinarily, I don't like to split up either -- I have an innate fear of not reconnecting, or of something happening to one of the parties in the half-hour they're apart!! I'll make an exception in this case, however LOL. The airtrain is easy (it's really the only option, other than a taxi, which is silly), it's close, and it's very easy to get back to the terminal once you leave the car rental (one freeway exit).

We have never split up & not had hassles getting our young kids & all our gear to the car rental place & back. It's really not as difficult as it may seem, especially if you can fit everything (except baby who will probably want to be carried) on one of the rented luggage carts). I still remember the days of lugging car seats and stroller & tons of gear for infant & toddler plus us two adults.

I guess it's a matter of preference. We always felt better sticking together. I'm sure that splitting up could work as well, but especially in strange cities, we sometimes get directionally challenged.

Splitting up has worked well for us when travelling with my elderly parents and wheelchairs and too much luggage to manage. As long as you have a cell phone to communicate, it is a simple matter to wait until the rental car arrives.

"No extra stairs or other obstacles between point A and B?"

There may be escalators - or you will have to find elevators. The presence of personnel is minimal, possibly nobody to ask questions.

Get a cart for luggage and stroller, should fit in an elevator. Will not be a problem on the train - it's basically an empty car with some poles to hold on, and a few seats in front and back.

"Even better would be to leave one person in the terminal with baby and encumbrances while the other person goes to get the car and comes back to pick them up." - not sure I would recommend this! How well the driver knows the airport to go to a terminal to pick up his family?

Perhaps they have improved things in the last few years, but 3-4 years ago we flew in (USAIR) and found the maze of walkways through parking decks to the train to be very poorly marked and confusing. There was a thread here about it on which non-locals agreed it was a pain, and locals (who of course have the benefit of lots of practice) saying it was a piece of cake.

There are elevators, but you may have to wait in line with all the other confused tourists.

Eli - we have been there done that ourselves - and as someone previously said finding someone on staff is very difficult - but there are elevators so that you can easily wheel anything into it with out hassles.