Optimal Table Structure
Here is a sketch for an Incremental Evaluation System (IES):

CREATE TABLE stops_min_max
( JournalPkey ... not null primary key
, MINStopPkey ... not null
, MAXStopPkey ... not null
);

CREATE TRIGGER ...
AFTER INSERT ON stops
BEGIN
        MERGE INTO stops_mean_max x
        USING (
            VALUES (__new.JournalPkey, __new.StopPkey, __new.StopPkey)
        ) y (JournalPkey, MINStopPkey, MAXStopPkey)
                ON x.JournalPkey = y.JournalPkey
        WHEN MATCHED THEN
            UPDATE SET (MINStopPkey, MAXStopPkey) 
                     = (LEAST(x.MINStopPkey, y.MINStopPkey)
                       ,GREATEST(x.MAXStopPkey, y.MAXStopPkey))
        WHEN NOT MATCHED THEN
            INSERT (JournalPkey, MINStopPkey, MAXStopPkey)
            VALUES (y.JournalPkey, y.MINStopPkey, y.MAXStopPkey);


Triggers for update and delete should be added

Instead of looking for min, max in stops, they can be found in stops_min_max

Since I have no idea what your tables look like, and I don't have access to the DBMS you are using, a sketch is the best I can do.