Hi all!
We all (should) know that not all Märklin coaches are made in 1:87 length scale.
I took some pictures to illustrate this.
Here's a picture of five coaches with a prototype length of 26.4 metres (UIC standard length):

Here we have:
- 24 cm class (1:110)
- 1:100 class (26.4 cm)
- 27 cm class (~1:97.8)
- 28.2 cm class (~1:93.6)
- 1:87 (30.3 cm (a Trix model, but announced from Märklin))
Here's a picture of five coaches with a prototype length greater than 26.4 metres:

Here we have:
- 27.5 m dining car, 24 cm class (~1:115)
- 27.5 m dining car, 27 cm class (~1:102)
- 27.5 m dining car, 28.2 cm class (~1:97.5)
- 26.8 m double stock coach, 1:100 class (26.8 cm)
- 27.3 m double stock control car, 1:100 class (27.3 cm)
The wooden bar indicates the prototypical length of the dining car.
Even shorter models came in shortened versions:

The old "Donnerbüchsen" were only 11.5 cm long (scale 1:121) while the new models are 16 cm long (1:87).
Here the side-view of the "Donnerbüchsen":

The old coaches have oversized lettering, so the prototype length is given as "13,9 m" while the new models are marked as "13,92 m" along with more information.
Here's a picture with 4 Am/Aüm coaches:

From top to bottom: Märklin 24 cm class, 27 cm class, 28.2 cm class, Roco 1:87
Please note that the coaches have varying prototypes, two have one-piece doors (with large windows), two have folding doors (with small windows).
Please note that the two short models only have 8 compartments while the long ones and the prototype have 10 compartments.
All coaches are lying with gangway to the camera. After I took the picture, I noticed that the frame details of the 27 cm coach were turned around - so I turned the coach and took another picture:

A closer look at the frame from below:

The frame of the Roco car is very detailed (bottom). The Märklin 27 cm car looks very similar (top), but nothing is printed or coloured. The Märklin 28.2 cm coach is in the middle (no comment).
The rear end of the coaches makes a difference with the last coach in the train. Here's a picture:

Märklin 24 cm coach: red rear lights are painted on, no window pane in the door windows.
Märklin 27 cm coach: painting the red rear lights is left as an exercise to the customer, no window pane in the door windows.
Märklin 28.2 cm coach: red windows for rear lights (can be made operational), window pane in the door windows.
Roco 30.3 cm coach: red windows for rear lights (can be made operational), window pane with coloured frame in the door windows, printed information.
A picture that allows comparison of the model window heights:

This picture also allows comparison of the interior details.
Roco (right or bottom respectively) seems to have the correct shape, details are coloured.
Märklin 28.2 cm class (next): no comment.
Märklin 27 cm class (next): shape seems correct, but details are not coloured.
Märklin 24 class (top): interior does not really match the compartment windows.
And one for width comparison:

Peeping through the windows (Märklin 24 cm class):

Three seats where there should be six, compartment door not showing.
Peeping through the windows (Märklin 27 cm class):

Sorry, picture is not too good (maybe I'll try again). Also see side-by-side picture above
Peeping through the windows (Märklin 28.2 cm class):

No comment.
Peeping through the windows (Roco 1:87):

You can see the coloured ventilation grill on the compartment door.
Märklin has mixed different length scales in several sets.
Maybe I'll take some pictures of such mixed sets to compare how they look side by side.
Update (June 1, 2014):
Märklin advertise their ICE 3 in the catalogue with the words "Scale length for 1:87".
When they sued Piko for plagiarism, they filed at court that both the Märklin ICE 3 and the Piko ICE 3 have a length of 1:88.8, a width of 1:88.57, a distance between pivot points of 1:92.8 and a height of 1:87.
Some details on the trucks are 1:100 with both models.
The product database still shows the "1:87" symbol for the ICE 3. It's too short by about two centimetres - the difference is way too small to be noticed with bare eyes, but length is rather 1:89 than 1:87 (along with various other compromises).
Update (October 20, 2021): Images now using SSL.
Edited by user 20 October 2021 20:57:22(UTC)
| Reason: Information about ICE 3. Updated image links to HTTPS.