Chiltern trains longer than 6 car on the MET line?

[CR165022]

So, something I've always wondered, is this not allowed? I only ever see 6 car formations being the longest on this route, however then on the other hand, I saw with my own eyes a 7 car formation a few years ago.

Asking today, because Chiltern are having to replace a 3 car 165 with a 2 car for the remainder of it's diagram, so that they can get a 4 car 168 down to Marylebone at the same time to replace one which failed this morning.
I understand needing to get a 168 down, however, a 3 car 165 being replaced with a 2 car one for seemingly no reason other than to get that 168 down too, isn't exactly fair to passengers? Especially considering said diagram goes to Oxford right after, one of the busier routes.

 

[Calpe]

6 cars is the longest formation allowed over the Met now for Chiltern stock.

 

[800002]

It's not allowed.
The LUL infrastructure isn't set up to deal appropriately with any more than 6 mainline vehicles, or their equivalent length.
Something to do with the track circuits, I believe.

It is possible to operate longer formations (it has happened) but it requires special arrangements.

 

[CyrusWuff]

Also Chiltern services stopping at Rickmansworth can only have five vehicles open, due to the length of the platforms. There's no space to extend them to allow six car trains to call either.

 

[sammyg901]

Realtime Trains | 1811 London Marylebone to Aylesbury | 15/04/2026

Real-time train running information for 1811 departure from London Marylebone to Aylesbury on 15/04/2026. From Realtime Trains, an independent source of train running info for Great Britain.

www.realtimetrains.co.uk

I was confused yesterday when I saw this as I thought max 6 too. Must have been a data error as in reality it was shortformed as a 2 car (bet that was cosy!)

The 1830 was showing as 5 cars vs 3 so I thought that was another error but it did actually run as a 5 with an additional stop at Amersham. Presumably to get the right number of units to Aylesbury. The attachment at Marylebone was well managed by the staff as they needed to keep the 3 car empty until the 2 car came in and attached

 

[800002]

Units can end up all over the shop quite quickly when disruption occurs.
Not ideal, however, having to wait to empty one unit before attaching another, though.
And it's all often hours after the original disruption has ended.