I began in the industry 15 years ago timetabling. We used ProTim which was a legacy system Railtrack used. The TOCs were all using Voyagerplan. All the schedules had to be manually entered and validated. Later systems incorporated an import function where the TOCs schedules were imported which saved on inputting them, but still needed validating.
As mentioned, a good planner could validate a path using a variety of tools. My favourite was junction reports for just a few trains. Knowledge was key, you needed to know your area. Where had loops, 4 tracks, platforms to thread these trains through. For larger schedule changes graphs were used with a pencil and rubber.
Planning rules had to be obeyed, signalling headway’s, junction allowances, engineering tolerances, platform dwells etc. Sometimes refining one train meant changes to several more just to accommodate it!