Is the railway train still running from Battambang to PP? If so how long does it take? I am keen to ride on the roof as the LP guide says the train can be rather crowded. Are there options to get off and travel part of the way by bus? Also any information on the Bamboo Railway would be most appreciated. We will be travelling in Cambodia in early December. Thanking you in advance for the info.

The train runs from BB to PP most Sundays. The only way to be sure is to get to the railway station on the Sunday you wish to travel.
Its awfully slow and unreliable. A guest of mine recently took it the 100km's to Pursat which is the next province over. It took 10 hours to reach the destination. For train lovers only!
The bamboo train is a tourist gimmick on the outskirts of BB. I hear they have been charging people as much as $10 lately for the ride. This is one trap that I don't recommend.

I did it. (I’m the guest mentioned in #1)
Yes, it’s slow, it’s dirty, it’s uncomfortable.
But: you get a glimpse of Cambodian life a visitor usually doesn’t get to see. And you also get a great and different view of the countryside and villages you pass through.
When the train left Battambang (at 7.25, only about 45 minutes late, not too bad), it was fairly full, although not crowded. Some men, who at once got a very lively card game started, a couple of young women, all dressed up, an elderly gentleman, in carefully ironed white shirt and beige trousers (obviously an optimist), carefully balancing a leather briefcase on his knees, and some market-women, along with a sacks and sacks of rice, pineapple and other vegetables. These sacks were stashed everywhere, including in the obnoxious little cabinet with the hole in the ground, meaning that the toilet was not accessible. (This is actually my main complaint about the whole trip. The men could go to the back of the carriage and let it go, and I suppose the sarong-wearing ladies did likewise, but for me it was grin and bear it….thank goodness they call me the Woman with the Bladder of Steel..).
I had read somewhere (on this site?) that it is unnecessary to bring food because there are so many vendors at the stations. It seems the other passengers had read this also, because nobody had any food with them. All Battambang station had to offer in terms of catering was an old lady selling sweet drinks, chewing-gum and Tiger Balm. There was also a stall serving noodle soup where I should have had a bowl, but I was hoping for the famous food vendors. But the few sorry little stations we passed had little to offer in ways of entertainment and even less of food. – And then, around 11.30, Moung Roussey, culinary highlight of the Royal Cambodian Railway! Smoked and fried fish, cooked rice, grilled taro, fresh fruit, deep- fried cakes, grilled things on sticks…At once the overall mood brightened up considerably. Everybody started chatting and joking, food was passed around, and suddenly our rather glum little group had turned into an actual community. The men resumed their card game with even more vigour and shouting than before. They also took off their shirts because the temperature had risen to sauna heights. This was an option not open to us women, so we had to follow the Victorian rule of “A true lady never perspires!” – Somebody had put a huge basket full of live ducks on board and their quacking added to the general mayhem. They also added an extra nuance to the already quite pungent fragrance…
The market women were by now obviously dying to know what I was doing on this train all by myself, and I would have loved to find out about their lives… I so wished I could speak Khmer.
And so our train rattled and shuddered and clanked (and quacked) its way through the lush Cambodian countryside. Every now and then the whistle blew, usually to warn people on the “bamboo trains” to get off the rails. They then stood by the rails, often in mud or brush, probably thinking “It had to come now at this dreadful spot, didn’t it!”.
The train reached Pursat about 4.45 and I was almost sorry to get off.
Would I do it again? – Yes, from Phnom Penh next time.
Would I go all the way? – I don’t think so. Hours and hours of rattling along in the dark doesn’t seem so appealing. So again to Pursat or Roumeas.
What would I do differently? – I would bring a hammock. Many passengers had one and it looked like a nice way to travel. And I’d bring some food. And I’d wear a sarong and no underwear.
(Btw: BBHelper has a great guesthouse!)

JAX
You cannot ride on the roof anymore. LP is out of date.
Did this trip 2 weeks ago and do it every other month, speak Khmer, know the conductor and still no go.
P.P. to Battambong Saturday 6-30 am leave. I go to Pursat and take a taxi to B.B. Much easier.
B-bong to P.P. Sunday 6-45 am leave but harder to get a taxi to P.P.
I left P.P. at 6-30, got off at 2pm, taxi to B-bong by 4pm and the train arrived B.B. at 12-30 am!!!!
I think the bamboo train is wonderful and recommend it more than the train itself!!
You can taxi from P.P. to Oudong and meet the bamboo trains there or in B.B. and do it there.
enjoy

My partner and I rode the bamboo train near Battambang two weeks ago after watching this video: http://www.youtube.com/watch?v=yjBYjAookl4 and talking about her experience with the video's maker in a Siem Reap bar. It was a total gas. I would higly recommend it.
Tranport to the train was arranged through local English-speaking moto driver named Kim Sok, (local cell #: 012 654 427 - email: kim_chhaya@yahoo.com). Kim was very cool and laid back, no pressure at all, just a really good joe.
The train ride itself is cheap ($3 for all 4 of us including the moto's between stations - about 10 minutes -- $5 for all of us to go two stations worth - about twenty minutes, we opted for the more expensive otpion).
We paid considerably more for the 3 hour tour $10 each, but we are both glad we did as we got a good look at local rural life around Battambang and had a good time. I'm sure if you pressed you could get Kim to charge far less just to get you out to the train and back.