On my two rail, the first group of 8 battery R/C locos I have are Piko simple basic $100. units. They use MTO batteries and are charged using external ports except for the Lionel 2 rail 2-6-6-2 which uses the flip up water hatch to access the charging port and on-off switch.
I find that I am only charging the units about once a month. I regularly rotate the 2 rail locomotives.
My SS RDC uses a discrete button and charging port on the under carriage as does my RS/11
The GP-7 uses a smaller battery due to obvious reasons. Slight dip in run time but well worth it.
All batteries are onboard, no trailer cars for the battery are used.
Most of my RR has been previously star wired for a dozen PS-2 & 3 locos running conventional. The wired mainline has the ability to run AC or DC in any power district with the flip of a switch for that particular area. I find PS units run best on AC.
Once I discovered battery R/C the wiring stopped. All subsequent track construction is to be used with battery only.
I have a Stenton system which is looking for a new home and plan to start with Rail Pro next week.
Am scheduled to pick up the first one or two Rail Pro kits next week to mock up speaker units to see which tenders will be better candidates to contain everything.
My three 3 rail lines are each wired, no thought to go battery there. I just turn on the three rail trains and let them run the entire time. Their sounds and travel motion add a lot the the overall model railroad presence. I got the idea of mixing disciplines from the Brandywine River Museum in Chadds Ford PA. I had forgotten about the nostalgic joy of three rail.
The three rail has long runs, point to point weaving through the scenery via return loops at each end. I fail to see any advantage to go battery R/C with the constant running three rail. YET !
All the while the two rail is running commuters and freight back into the countryside on a variety of branch lines from the mainline interchange to serve an assortment of industries and public delivery tracks. My "operation 101" process is basically a Cliff Notes version. True operation function reminds me too much of work. I'm retired.