In my mind, historical is like being pregnant, you either are or are not.

And I deal in Historic Restoration as part of my profession, so I'm not sure what criteria to use here.

As to this "oldest" question, the B&O was first railroad in th US to cmplete all of the following,

Be Chartered or Incorporated for the express purpose building a railroad.

Actually build a line that went somewhere and provided regular service, not just a novelty or experiment.

Stayed in business to actually provide such services as a successfull busness.

They then followed that with a long list of industry "firsts",

first published timetable,
first stone arch railroad bridge,
first double track line,
first train to exceed 30 mph,
first railroad to pay a dividend,
first to operate a railroad repair shop,
first locomotive turntable,
first baggage car,
first railroad to Washington DC,
first to carry US Mail,
first iorn box car,
first dining car,
first to reach the Ohio River,
first to use an electric loco,
first to use a mallet loco,
first air conditioned car,
first completely air conditioned train.

Now that is Historical!

Sheldon

                 

And first in the number of bridges burnt or otherwise damaged by Colonel John Singleton Mosby, commanding officer, 26th Virginia Cavalry, CSA.   
Gene

Yeh,

And that too!

Sheldon

Yorkie - we have conveniently omitted the fact that Britain had an operating railway before one started over here. I remember reading in grade school about how the "John Bull" locomotive (British built) had to race a stage coach.

As for most historical I think the votes are counted and the "Beano" wins.

I don't remember anything about a race involving the John Bull.  The most famous "race" was on the B&O (again).  In the beginning the B&O was operated with horses.  Peter Cooper wanted to convince the management that steam power was the future, and cobbled up a small engine on a shop cart.  The whole contrivance probably wasn't more than 6 feet long.  Cooper took some folks out on a demonstration run towards Ellicott City.  On the way back, they apparently came upon one of the horse-drawn cars and a bit of friendly competition briefly ensued.  The belt that ran the draft fan came off its pulley, and the loco (now known as the Tom Thumb) lost steam and had to stop for repairs.  When Edward Hungerford wrote his "history" of the B&O for the 1927 Fair of the Iron Horse, he somewhat inflated this incident into a minor epic. 

I would note that the "replica" built by the B&O for the Fair is way larger than the original.  Mel Thornburgh, a modeler who did a number of construction article in Model Railroader in the 1940's and early '50's, built a more accurate model of the Tom Thumb which is on display in the B&O Museum in Baltimore. 

Another side note:  Anyone else on this list see the original John Bull in operation?  Not the replica they have at Strasburg.

OOOPS - I forgot about Tom Thumb. The story I heard was about a race with Tom Thumb and a horse - not John Bull.

There are lots of railroad myths out there and the details of the"race" may be among them.

For a final "B&O" plug I suggest a visit to the B&O museum in Baltimore. I understand that the roundhouse was damaged or burned down in a fire. Is that true?

Woody,

Part of the roundhouse roof collapsed from snow several years back. It is all beautifully restored now and many upgrades to the whole museum where done as well. Several locos and rolling stock that where damaged are still in the restoration process or awaiting money and/or their turn in the shop. But the museum has a vast collection and is well worth the trip.

Sheldon

What ever to GG-1 #4876 that they are supposed to own? It was sitting vandalized in a CSX yard as recently as a few years ago, but I've heard rumors that it's been moved. I hope it has- 4876 was the one that crashed into Washingto Union Sation in 1953.