Here's another circa 1900 culvert, this time close enough to see the that the mortar has deteriorated. There's a treasure trove of wild raspberries to be found here.

With separate arches for the roadway and Patapsco River, the Twin Arch Bridge is one of the few places you can drive under the Old Main Line.

The Twin Arch Bridge shown here dates to 1901 when the tunnel was constructed to bypass the old, steep and winding Mt. Airy Loop route around the north side of the town.

But, this is not the first Twin Arch Bridge near Mt. Airy... stay tuned as you scroll downward.

A junkyard resides between the Mt. Airy cutoff and B&O's original inclined planes alignment.

Pushers that assist heavy eastbound coal trains often detach in this vicinity, though that is not the reason for the signals. Noisy I-70 traffic zooms past behind the trees on the left.

This Equipment Defect Detector at Ridgeville checks for dragging equipment (between and outside rails), and hotboxes (outside rails) monitor temperature in case excess friction is causing a wheel to overheat. After a train passes a report is automatically broadcast via radio signal.

Link: sound recording of detector at mp 18.1 (Daniels) (340K WAV file)

The summer sun is never directly overhead in this region and thus will never confuse the sky-looking hotbox. A guard on each side deflects dragging materials that could damage the box.

See, every so often there *are* trains that run on the OML, even when a camera is handy.

In a moment this eastbound coal drag led by CSX 441 will turn the then-new signals at mp 38.8 red.

These abandoned bridge supports once carried US 40 across the OML. Unseen but audible on the left, the rerouted US 40 now shares a path with Interstate 70.

There's also an older disused bridge to be found ahead...

Hidden in the brush is the only surviving part of B&O's first Twin Arch bridge near Mt. Airy. It was built in 1831 as part of the original (inclined plane) route over Parr's Ridge. The arch seen here carried the railroad over a small stream. A larger adjacent arch, long ago removed, had spanned the Frederick Turnpike that connected Baltimore with the early 19th century National Road to the west.

From the start, B&O knew the road here was busy enough to justify a bridge for grade separation, one of the first railroad grade separations in the world.

Time has not been kind to the original Twin Arch bridge, as can be seen from the opposite side. Unlike its circa 1900 counterpart, this bridge was crafted with precise stone cutting so as to require no mortar.

Springs from a high water table under Mt. Airy bedeviled railroad construction. Mt. Airy Tunnel peeks from beyond the bend. Nearby, disused stone supports tower overhead where decades ago they had carried Ridge Avenue over the tracks to what is now Bennett Branch Road on the south side.

At about a half mile in length, the Mt. Airy tunnel is, by far, the longest on the OML. It is almost twice as long as the Ilchester Tunnel, second longest on the route. The tunnel is practically in the shadow of Interstate 70, but you'd never know it because the tracks reside deep in a cut made into the ridge. The other portal is not visible because the tunnel curves slightly (to the right, in this view).

The tunnel dates to 1901 when the Mt. Airy Cutoff was constructed to bypass the old, steep and winding Mt. Airy Loop route around the north side of the town.

Near the top of the ridge, Parrs Spring represents the headwaters of the Patapsco River. The stone added in 1976 celebrates the only location in Maryland at which four counties meet: Howard, Carroll, Frederick and Montgomery. The actual meeting spot is the spring at the pier.

Related trivia question: How many roads cross into Howard County from another county, without in the process spanning a river or railroad?

Answer: Only one: Ridge Road, at a location about 1000 feet southwest of the four-county intersection seen here. The road manages to squeeze through a tiny gap between the headwaters of the Patapsco and Patuxent rivers.

Here's Mt. Airy Tunnel's west portal; the tunnel was opened in 1901. If you visit this area during the summer, take note that the brush is loaded with deer ticks, carriers of Lyme disease.

Bush Creek, the stream the OML follows west from Mt. Airy to the Monocacy River, meanders under the railroad for its first time. Limited access prevented me from getting a better picture of what is the first stone arched bridge to be found on the route west of Mt. Airy. It appears that 29E is the painted-on bridge number.

Bush Creek briefly snakes between the OML and the Mt. Airy Loop, then dives under the latter. This bridge is substantially older than 29E pictured above, dating to the 1838-construction of the Loop.

Look closely above the debris and runaway furniture and you'll see that the original voussoir (stone arch) was retained during a rehabilitation and addition of metal pipes to carry the creek.

To my knowledge, this is the oldest bridge with surviving portions still in railroad use west of Woodstock (back at mile 22).

This weary CPL signal that has stood guard for more than 50 years where the OML (left) and Mt. Airy Loop meet. On each side, a new generation of signals waits their turn. Will they be as durable?

Old signals don't die, they just rust away...

Most of the OML's CPLs were installed in the early 1940s to implement Automatic Block Signalling to handle the increased rail traffic of World War II. Prior to that, semapahore style signals and manned towers had presided. Some of the semaphores were reinstalled at other spots and used until around 1960.

I believe the 4-lamp CPL signal in my photo here is the same one in the 1982 photo linked below.

Link: 1982

Last call for drinks from this B&O water tank came during the 1950s. It had likely been installed around 1901 as part of the Mt. Airy Cutoff. It is located between I-70 and the railroad at the western limit of the Mt. Airy track.

Something I find interesting is that steam power is still used in modern electric power generating facilities: even nuclear power plants boil water to create steam that turns generators.

This bridge is notable because it is the OML's newest, built around the time I-70 was (early 1970s). I-70's bridge can be seen in the distance.

During that construction, the railroad alignment was shifted slightly north (left) of the original. The flat area on the right marks the previous location of the tracks.

A climb (slide?) down a very steep embankment was rewarded with this fine stone arched bridge. The interior arch is made of brick. This was constructed around 1901 at the time the Mt. Airy Tunnel was bored; simultaneously the railroad rebuilt about 10 miles of track in the vicinity of the tunnel.

Bring your crampons if you want to scale the embankment back to the top.

This is the other side of the same bridge.

At first I wondered why the "tunnel" for the stream is so long. I checked maps and noticed that the construction of I-70 necessitated a small realignment of the railroad. The tracks were shifted north, and rather than destroy the old bridge, the railroad decided to extend it and retain the arched style. 1973 is embossed into its concrete.

As a result, to my knowledge, this example represents the most modern (and perhaps last) arched masonry bridge construction built by/for B&O.

Near here had been the bottom of Plane 4, the westernmost of four steep, inclined planes the railroad employed during the 1830s. At each plane, first horses, then perhaps steam winches, then helper engines were used to boost trains up over the ridge at Mt. Airy.

Several people reported this stone arched bridge is visible from I-70. In fact, if you look through the arch you can see the end of a tractor trailer on that interstate.

This view, courtesy reader Terry Iman (thanks, Terry!), is from the end of Clear Spring Lane. It would appear that before I-70 bisected it, Clear Spring had been part of Woodville Road, which can still be found on the north side.

The mortar indicates this is not an original 1830-era bridge. It was constructed as part of the Mt. Airy Cutoff (tunnel) project.

Of this structure, reader Tom Thornhill wrote:

I have thoroughly enjoyed your tour of the old main line. The photos and information have been great. However, I noticed that in your Mt Airy to Frederick section of the tour you were unable to locate an old stone arch. I know of an old stone arch bridge that is between the Moxley Road Bridge and Lynn Burke Road. The stone arch is just south of Plane Number 4. Before the up grade of I-70 in the early 70's the stone arch was used for a local road to access the farms south of the railway. With the up grade of I-70 the road was abandoned.

Vaguely visible during non-leaf season from eastbound I-70 if you know where to look (just before Bartholows Road) was this sign. "First 1000 track miles continuously welded rail Aug. 1961 to Nov. 1965 Baltimore and Ohio Railroad Company." Someone removed the sign during the final 4 months of 2017.

Continuously welded rail, aka ribbon rail, eliminates the clickety clack of the gaps between typical 39-foot pieces of segmented rail. Those clickety clacks slowly take their toll on voth the rolling stock and trackbed. Continuously welded rail must be laid with care since subsequent expansion due to heat can cause the rail to buckle. A temperature increase of 80 Fahrenheit degrees would cause a mile worth of rail to lengthen 4 feet. Therefore ribbon rail is usually laid during hot weather because the steel and ties are better able to resist the contraction caused by subsequent cooler temperatures.

Even so, in this region CSX usually issues a "heat order" when the air temperature exceeds 90F (32C). A heat order decreases the speed limit for trains on the basis that a lower speed puts less stress on the rails.

Thermal expansion like this is being used to power a clock for 10,000 years.

Link: Long Now 10,000 Year Clock