I believe the current plan is for Tri-Rail to run every other train to downtown. So about every hour during rush hour and every two hours during off peak/weekend. Not ideal, but better than nothing.
My understanding is that Tri-Rail intends to set up a more complicated tangle of service...but (per mentions of doubling ridership or more abound) you'd add a
lot of trains to the mix. About half of the trains originating up in Magonia Park or thereabouts would run to downtown, but there's another "line" that originates at Pompano that would also run on the current tracks. Additionally, those trains being redirected into downtown are on a third "line" originating further north (up in Jupiter), which probably means a few more trains there as well. And all of this is aside from the FEC-only trains running from WPB down to Miami.
If the ridership projections in the 2011 report are anything to go by, what you have is as follows:
-FEC Local (Magonia Park to Downtown) 4 TPH peak/2 TPH off-peak
-Seaboard Flyer (present service) 1 TPH peak/1 train every two hours off-peak
-Flagler Flyer (Jupiter to Downtown) 4 TPH peak/2 TPH off-peak
-Airport Flyer (the Pompano-originating service) 4 TPH peak/2 TPH off-peak
So in total, downtown would see 8 TPH peak/4 TPH off-peak while the airport would see 5 and 2.5. If the plan were to stick, it seems quite plausible that FEC would push to add Pompano to their system (at a bare minimum, since that station would serve all three high-frequency lines and be sending out Tri-Rail trains every 5 minutes at peaks...the transfer ridership from the Airport Flyer and the local stopping trains would probably be substantial), especially since AAF wants to reserve the right to add stops in South Florida. Considering the sheer amount of time the trains will take from end-to-end compared to AAF, I suspect that you'll see a lot of transfer traffic feeding in...particularly around WPB.
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On train counts, those TPH numbers would probably work for Tri-Rail. With AAF, a lot depends on where they handle the servicing. Also, while AAF is saying "hourly service", if their numbers are any indication there are strong hints of planning for up to four AAF trains per hour: First, their ridership projections don't quite work with once-an-hour service (I ran the numbers a while back and you couldn't jam enough people onto their trains for the numbers to fit, especially south of WPB). Second, they've explicitly wanted to plan with Tri-Rail for up to 64 trains per day. Third/finally, they seem strongly inclined towards adding Jacksonville in reasonable order (doing so is probably cheaper than Orlando is, even if ridership is lower) and adding JAX would inherently add pax (yeah, I couldn't resist)...and with it, more trains.
So my guess is that, assuming all of this holds, AAF+Tri-Rail would probably be looking at 10-12 total TPH into Miami Central Station down the line. Tri-Rail's 8 TPH should be fine on two tracks as long as the track layout works well to avoid bottlenecks into/out of the station (I'll grant them this much). AAF is the more complicated customer, depending on how AAF wants to handle restocking and how long they want to be able to have the train in the station for people to board in advance. Additionally, if they service the trains elsewhere, that means additional movements in and out of the station.
However, I suspect this much traffic is probably going to mean that a few stations (Pompano comes to mind) will need to go to 5 tracks (I'm thinking two north, two south, and one through track without a platform for freight). I don't think you'd need more anywhere but Pompano (where the turns of the Airport Flyer become an issue...there you have 8 Tri-Rail trains running through, 4 terminating and turning, and then AAF doing...whatever AAF chooses to).
Edit: There appears to be a third connector track in Miami in the plans, albeit without much stated use...so it is quite possible that
another "line" might come into use (probably Magonia-Downtown via CSX almost all the way).
