train from Riomaggiore to Venice

Hop on the train to La Spezia then connect through Milan. Easy simple. I don't believe the train that leaves from La Spezia stops in Rio going northward.

Before you follow the advice given above, I suggest you go to www.trenitalia.com and enter the following:
From: RIOMAGGIORE
To: VENEZIA S. LUCIA
Time: a little before your expected time of departure, for example 10.
Date: enter a date in the next week ( regional timetables are loaded weekly:
Seach

All the available options will display. Probably, none of them will require you to go to La Spezia to catch a train to Milan. That would be backtracking, which you could easily avoid by catching the same train in Levanto. Also, going through Milan isn't necessarily the fastest option.

You can go thru Levanto or La Spezia. Two possible current schedules (based on traveling tomorrow):

12:33 Riomaggiore to Levanto
13:01 Levanto to Milano
16:05 Milano to Venezia S.L. arriving 18:40

OR

12:55 Riomaggiore to La Spezia Migliarina (not Centrale)
13:42 L. S. M. to Firenze
16:30 Firenze to Venezia arrive at 18:35

You can further research options on the Trenitalia website. Be sure use Italian city names (Firenze not Florence, Venezia not Venice).

Its a long trip. I did the reverse from Venice to CT last year. I took the earliest train out of Venice and slept the first couple hours. Made CT in time for lunch

Thank you, all!

We did the reverse and I looked at schedules to go through Milan vs Florence -- the latter made more sense to us for the departure/arrival times we wanted, but I think the overall journey time was fairly even between the two.