What about the Door(that wasn't at the end of the tunnel)?

[potioncat]

No: HPFGUIDX 99780

Jessica wrote:
  Snape  was able to use the tunnel as a child and an adult, Lupin 
uses it as  an adult, Harry, Hermione and Ron use it as well, so it 
must be big  enough for at least an adult to use (although it may be 
cramped). 
> But I do think a stag would have a hard time using the tunnel if a 
> person had to bend over to use it :))
> 
> Although there is no proof against an enchantment to prevent 
> Transformed!Lupin from accessing the passageway. 
> 


Potioncat:
The part that makes me think that the werewolf didn't normally get 
into the tunnel (without Marauders' help) was Lupin's comment 
that "had Snape gotten in the house, he would have met a full grown 
werewolf."  (paraphrased)

I think everyone except James could have used  the tunnel, as an 
amimagus.
Potioncat