I would preface my comments on this subject with the note that India is a vast country with a vast rail,network that is ever growing in size and sophistication. One can spend a lifetime visiting various corners of it as some of my railfan friends in the IRFCA do.
Any advice on particular lines or features of interest (in terms of scenery, equipment, other remarkable features etc). Ideally I would like to get a taste of both the old and the new. Also hopping off the train to see places of cultural / historical interest if they can be easily combined. Though I'm not much of a fan of the well-organized super tourist traps and would prefer more second tier attractions that still have a lot of local ambience.
In New Delhi visit the National Railway Museum in Chanakyapuri to get your fill of remarkably well preserved historic artifacts and rolling stock, including a Class N Garratt and a bunch of DC electric locos from the 30s and 40s used around Bombay. There is also a well preserved Baldwin WP bullet nosed passenger steam loco too.
Inquire at the NRM whether it is possible visit the Steam Loco preservation shed in Rewari a relatively short train ride from Delhi, and if available visit it.
The scenic day trips from Delhi are mostly to the Himalayan foothills - Dehra Dun and Kathgodam (for Naini Tal) have Shatabdi Expresses. Overnight trip to Katra on the part of the Kashmir Rail link that is in service from the plains would be worth it. The other end of it in Kashmir Valley can be ridden between Banihal and Qazigund, through the Pir Panjal Tunnel under the Pir Panjal Range (foothills of the Great Himalayas) can be done by flying from Delhi to Srinagar and spending a day or two there. Do this only if you have a stomach for the possibility of working around occasional terrorist activities, though it appears to be on the wane now.
On the trip to Katra, you could stop off at Pathankot for a few days and take in the narrow gauge Kangra Valley Railroad to Joginder Nagar and back.
Also on the way back stop at Ambala Cantt. and spend a couple of days going upto Kalka and then taking the Kalka - Shimla narrow gauge line up into the Himalayan foothills to what used to be Summer Capital of the British Raj.
A few of the not to be missed scenic lines on the plains are:
1. Kokan Railway -there are through trains from Delhi (Hazrat Nizamuddin) to Tiruvanthipuram, including a Rajdhani Express to explore this line along the west coast of India where the Western Ghats meet the Arabian Sea. Make a stop at Mumbai and take in the ride upto Pune and thence to Madgaon (Goa) and then join the train to Tiruvanthipuram there in one directio to experience the 1 in 37 Broad Gauge climb climb from Karjat to Pune and then the sceneray of the Sahyadri range of the Western Ghats. Stop off at Goa and enjoy the beaches and the hedonism
2. From Tiruvanthipuram go to Chennai across the peninsula, and then catch an Express (perhaps Coromandal Express) to Kolkata (Howrah) to experience theEast (Coromandal) Coast
3. From Kolkata take the daytime Shatabdi Express to New Jalpaiguri across the might Ganga at Farakka Barrage.
4. Overnight at New Jalpaiguri and then catch the early morning Narrow Gauge Train to Darjeeling on the UNESCO World Heritage listed Darjeeling Himalayan Railway. If you are lucky you will get a spectacular view of on of the top five highest peaks of the world (Kanchenjunga). If you are even lickier you could get a view of distant Mt. Everest. Tiger Hill is the place to visit early in the morning for the best views.
5. From New Jalpaiguri catch a train to Agartala and go at least as far as Badarpur in order to take in the spectacular Barak Valley segment between Lumding and Badarpur. Enroute, you will cross the might Bramhaputra near Guwahati.
6. Now there is a direct Rajdhani from Agartala to New Delhi which you can take to hightail it back to Delhi from this eastern extremity of India. There are many areas in and around Assam state and the surrounding border areas where you need a separate Inner Line Permit, but that should not be needed if you stay on the main railroad all the way to Agartala, or on the norther branch out of Lumding to Tinsukia on the Bramhaputra.
7. From Kolkata you could take a side trip to Dhaka in Bangladesh by the international non-stop Maitrye Express. Need less to say you will need a Bangladesh visa endorse for land border crossing to make this trip, and a multiple entry Indian Visa.
8. While in South India a trip to Mettupalayam from Chennai to take the Meter Gauge Nilgiri Line to Ootacamund is highly recommended.
That is what I would do if I had a couple months to gallivant around on trains With that much riding to do, I'd spurge for a AC 1st Indrail Pass.
How best to book tickets? Online? At stations? How far in advance?
You can book everything on line at the IRCTC website. It can be a bit of a pain to get yourself set up with an account. In the past they had difficulty sending an SMS to a number outside India in order to provide an OTP to establish verify your contact point. I think that is fixed now, so it should be easy.
Be careful with your baggage. They sometimes have a tendency to go walkabout while you were not looking, though less so from AC 1st Class, but all trains do not have that accommodation. Most common is AC or non-AC 3-Tier Sleeper or Chair Car (day trains). Even though reserved, there is a tendency for extraneous additional people to get on for short rides in those.
I gather from googling that India doesn't really have the concept of sleeper trains as in cars with private rooms with doors you can lock as on Amtrak, but what they call sleeper trains are more like communal dorms. Is this correct? Do they have showers on such trains?
AC or non-AC 3-Tier is communal dorm. AC 2-Tier is like Sections with fully curtain enclosed berths at night. AC First Class is shared compartments with lockable doors.
What is photography like in India. You occasionally hear scare stories of photography being absolutely forbidden in many locations, and of security staff and police acting allergically to anybody who even looks as if they might be planning to take pictures. Is this still the case? Any specific etiquette I should be aware of?
Things regarding photography are much more relaxed these days, is what I am told by my friends who do a lot of railroad photography in India.