Is there anywhere online with interior layout plans?

[Crithylum]

Is there a place where I could find diagrams showing the internal layout of trains, such as seat, toilet, and door positions? It would be interesting as it could be used to make quantitative comparisons between the interior “efficiency” of various trains (eg exact numbers for how the voyagers piss away capacity).

 

[Supercoss]

British Rail had diagram books back in the day, many of them reproduced here :
www.barrowmoremrg.co.uk/Prototype.html
And some Mark 3 carriage detail

Mk3 passenger coaches for hire, railtours, filming and test trains

ROMIC Group have a wide range of Mk3 passenger coaches available for charter and hire for a wide range of operational requirements.

www.romicgroup.com

 

[Scottish-Exile]

See the pdf for the TfW fleet: https://tfw.wales/info-for/passengers/accessible-travel/train-accessibility

 

[wellhouse]

Not sure how comprehensive these are, especially with so many fleets undergoing refurbishment, but Man At Seat 61 has seat maps;

Train seat maps | Seat numbering & layout in European trains

Seat Guru for trains? This page lists seat maps showing seat layout and numbering for European trains including Eurostar, Thalys, TGV, ICE, Frecciarossa, Intercity, Lyria, Thalys, Nightjet and more.

www.seat61.com