For midi (or audio I suppose). When composing I need to just select a track to hear/play on it without having to manually switch monitor and record. How can this be done automatically in Reaper by track selection? Thanks.

Automatic record-arm 

Right click on the monitor icon and select Automatic record-arm when selected. Then set the monitoring to auto.

Thanks. I got the first part (cool) but the second part isn't following the selections. And yeah, most of my time with Reaper over the last few years was basic editing etc so I have to "circle back" on some of this stuff. Appreciate the tip.

Set all your tracks to monitor input by default under: preferences > project > defaults > record config