Trains arriving simultaneously at Harrow-on-the-Hill

My experience is that they don't arrive at the same time, but due to driver changes, the slow and fast trains may sometimes be on platforms at the same time anyway, allowing for interchange. If two slow trains are on both platforms, the drivers usually do a good job in informing passengers of the next departing train. 

For southbound morning peak journeys on the Met line, many people will get off the slow train on platform 5 and wait for the fast trains on platform 6. If both a slow and fast train are on platform 5 and 6 respectively, an interchanging passenger will save around 3-4 minutes by the time they get to Finchley Road, which may be higher if they need to use the Jubilee line and are also able to immediately catch it at Finchley Road. My experience is that the drivers of most Jubilee Line trains do not bother waiting to allow interchange. This is probably because the Jubilee Line is highly frequent, about every 2 mins, so the wait isn't long. On the northbound journey, people will often wait for the fast/ semi fast services at Baker Street/ Finchley Road, even if it saves only 2-3 minutes because of the stop at Wembley Park. However, some people will also get on board an empty train at Baker Street (Platforms 1 or 4) for a seat. 

All Metropolitan Line trains stop at Harrow-on-the-Hill (HoTH) without exception. A few peak Chiltern Railway services (going into London in the morning and out of London in the evening) do not call at the station. Not many people in my experience, who are travelling southbound into London in the morning, will get off a Met line train to catch a Chiltern train, presumably because most want Baker St or Jubilee Line services. Assuming a Met train arrives on platform 5/6 going into London, if someone uses the subway to then catch a southbound Chiltern railway train, with minimal waiting time, this saves about 8 minutes getting into London I believe, but the Chiltern trains sometimes have a delay finding a vacant platform at Marylebone. You also lose some of the time, particularly if the train terminates on platform 6 at Marylebone, because of the long walk to the gate barriers. I don't know how many people get off a southbound Chiltern train at HoTH to get a Met train into central London, I suspect not many. Regarding the northbound services in the evening, a large number of people do take the Chiltern trains, and get off at HoTH, either to exit or interchange for platforms 4/5 into Uxbride/ Watford branches. 

If everything's running to time, none on the northbound.