Can anyone comment on what train to take from Venice going to Modena ?
Take any train south to Bologna. At Bologna alight the train and take a train to Modena.
Modena is on the Bologna to Milan old line (non high speed). The trip is just over 2 hours to 2h45m, layovers included.
www.trenitalia.com
Note that the Bologna station is huge and multi-leveled, leave a cushion of connection time (I usually try for 15 minutes).