Answer: Gravity Man

I asked a few questions about what a "gravity man" would do...  

1. What did a gravity man do? Where did he do it in California?
2. Is there any place you can ride a gravity car now?
3. Can you find a video of people descending in a gravity car? 

4. Can you find a gravity car that was used for serious work?  Where were they located, and what were they used for?  (Once you find the answer to one piece of this, the rest will come for free.)  

How I found my answers:  

First, I realized that a phrase like "gravity man" was probably relatively rare.  (At work we'd say it's "low frequency," meaning it's rare.)  

So I did the obvious first search to figure out what a gravity man was! 

     [ gravity man ] 

... and was surprised to learn that it's not rare at all!  There are all kinds of Gravity Man hits!  This is such a common phrase that just quoting the phrase isn't going to make any difference.  

But the other questions indicate that the "gravity man" was somehow associated with a "gravity car," so lets try that as an angle.  

     [ gravity car ] 

This gives much better answers.  The first organic web result is to the "Friends of Mt. Tam" and a description of their Gravity Car.  ("Tam" is short for "Tamalpais," a well-known mountain just north of San Francisco.  The drawing of the mountain on the question page is of Mt. Tam.)  

On that page, you'll find that the "gravity man" was responsible for "turning on the gravity," a clever turn of phrase that just means he released the engineless train down the track.  On Mt. Tam, it would go 8.2 miles from the summit to the station at the base.  With a 281 curves, and a top speed of roughly 15 mph, this was a great ride.  

Mt-tam-gravity-railroad_bw_April-13-1919i.jpg‎

Mt. Tam gravity car ca. 1915. Image from Wikimedia.

I clicked on a few other links from the SERP and on the FoundSF.org site is link to the YouTube video for one of the known films of the Mt. Tam Gravity Car ride.  

This video from 1917 shows a pretty spectacular hotel and recreation area on the summit (now long gone), as well as every man wearing a hat.  

I decided to go check Google Books for the Mt. Tamalpais gravity car, and was rewarded with a discovery of the book "The Crookedest Railroad in the World."  Unfortunately, it's not available for previews in Google Books (nor Amazon), so I turned to my backup book site, the HathiTrust, and looked for it there.  Success!  

An example of reading a book at the HathiTrust.org site.  This is page 76 of the "Crookedest Railroad.."

While looking for other books about the Mt. Tam gravity rails, I discovered the book "Mount Tamalpais Scenic Railway."  (By Fred Runner, 2008)  While skimming through that book, I found that Sir Arthur Conan Doyle (the author of Sherlock Holmes books) had ridden on the gravity car and written about it in his memoir about traveling the US ("Our Second American Adventure.")  About visiting Mt. Tam, he wrote: 

"There is Tamalpais, the one and only Tamalpais, which should be ascended by the traveler if he has only one clear day in the city of the Golden Gate.  Our party went up it on the day after our arrival, and we were agreed that in all of my wanderings, we had never had a more glorious experience."  

2. Is there any place you can ride a gravity car now?

In some sense, ANY roller coaster is a gravity car, at least according to the notion of a "train car going downhill without an engine."  

Luckily, our FriendsOfMtTam web site points out that they have a gravity car on an 84 foot (25.6 m) section of track.  That's not long, but you can get a tiny sense of the idea.  (Just remember to wear your hat!)  

However... 

4. Can you find a gravity car that was used for serious work?  Where were they located, and what were they used for?  

 The Wikipedia article on "Gravity Railroads" (found a few links down on the first SERP) gives the special additional sense of a "a railroad on a slope that allow cars carrying minerals or passengers to coast down the slope by the force of gravity alone."  This qualifies as "serious work."  

And this article points to several other gravity car lines, including a couple in Pennsylvania.  

Although they no longer exist, at the time the Mauch Chunk Switchback Railway was a technological marvel, attracting visitors from far and wide (including Thomas Edison and Ulysses S. Grant).  Built in 1827 to carry coal down the mountainside in northeastern PA, it quickly evolved into a tourist attraction on the weekends.  This gravity rail became the inspiration for the 1884 "Switchback Railway" on Coney Island, the first purpose-built entertainment roller coaster (in the US).  

Not far away from the Mauch Chunk was another gravity line,  The Delaware and Hudson Canal Company ran an extensive gravity railroad system from 1828 until 1898. The 55-mile (88 km) Pennsylvania Coal Company Gravity Railroad was the longest such line, and operated until 1885. In 1886 it was purchased by the Shohola Glen Summer Resort (1882) and used as an entertainment ride until 1907 (just before the Mt. Tam ride was set up).  

Oddly enough, the Delaware & Hudson gravity cars would drop their coal loads into the Delaware & Hudson canal boats, which would they carry the coal downstream to the cities of the northeast.  

But because the canal boats had to cross several rivers at a different height than the river itself, four aqueducts were created to carry the boats OVER the river.  These aqueducts were designed and created by John Roebling, the man who would go on to design and build the Brooklyn Bridge. He also is credited with popularizing the use of wire rope in suspension bridges (e.g., the Brooklyn Bridge), and ultimately in the Golden Gate bridge--which you now have to cross to get to Mt. Tamalpais.    

Search lessons:

1.  Know the HathiTrust.org site.  Keep them on your short list of places to search for books.  They often have full-view versions of books that aren't available anywhere else.  

2.  When the first query (e.g., [ gravity man ] ) turns up with way off-topic results, don't panic.  Just change your search to another target of opportunity.  As always, be aware of other phrases ("gravity railroad") and terms that are specific to your search.  

3.  Pay attention to small surprises along the way.  I stumbled over the Conan Doyle connection purely by accident.  These are great fun.  Put them in your notes for later use.  

Search on!