The problems with Welch, without more info are:
1: Did he have access to a vehicle.
2: Did he have friends that lived in the immediate neighborhood or could he sleep somewhere in the immediate neighborhood close to Drumm Ave and were his hosts away at the time
It seems to me that if 1 and 2 are not true, it would be really difficult to pull this off. I will say that back in 1975, there were fewer houses and streets in that neighborhood and there were homes under construction providing numerous places for disposal, I'm sorry if that sounds harsh. I was the same age as Kate at the time and lived in Colesville. Seems to me the girls are still in the neighborhood or they were taken away by vehicle. Welch has never been suspected of murders so does he really fit? Don't serial criminals find themselves unable to stop killing once they start? I can't really imagine Welch just coaxing two young girls that are supposed to be home by 4pm, talking them into walking down the RR tracks to Forest Glenn, Silver Spring, or Rockville for that matter and then molesting them and killing them.