Bonjour mon ami,
Let me make some corrections in the responses and posts above by fellow posters.
There's a train from Zahedan to Quetta, operated twice a month, you could check the schedules at their website.
The best and safest option for you to take a train from Quetta to Lahore, avoid the bus trip not safe, it takes some 22 to 24 hrs from Quetta to Lahore.
Please avoid any detour to Karachi, it is one of the most dangerous cities in Pakistan since January more than 1500 people have been perished in the unrest and target killings.
Dress properly, your attire is your safety, wrap yourself in a shawl, wear local dress (Shalwar, Kamees and Shawl).
In the train, try to sit with a family, it is safer and families in Pakistan are caring.
Avoid the busy markets, places due to chances of groping.
For stay please follow your guide book (lonelyplanet, footprints)..
Zahedan, Taftan, Quetta, Sukkur, Multan and Lahore, that is your route through the train.
I welcome your queries..
The railway system in Pakistan was built by the British, and the lines laid at that time still exist, with few additions added by Pakistan railway. Tracks total length is approx. 5000 miles (8000 km). The system is a life line for mass transportation of people and mail from Punjab/NWFP to Karachi and other cities. Special trains also run during long holidays, such as Eid. Unfortunately commercial aspect is not capitalized to the max whilst majority of the goods are still transported via trucks/roads.
Pakistan railway is government owned with Ministry of railway running its affairs. The rail system transports more that 70 million people annually. There have been talks of building fast-bullet trains, especially to cater Lahore-Islamabad cities.
The highlight of the train system is the Steam Safari train, pulled by two steam engines on each end, that runs from Peshawar to Landi Kotal, criss crossing various gourges and passing through several tunnels. I believe this is the only rail line that intersects a major international airport runway (at Peshawar). This train journey is an experience well worth exploring!
There are daily trains from all major cities connecting to different satellite towns and villages across Pakistan. For more details check Pakistan railway website.
Don't do it. Unless you want a real adventure. The train we were on was an overnight train that travelled from Lahore to the south. Our destination was Rahimyar Khan, about half way through the country. We travelled in what was called first class accomodations.... There is no glass in windows, only bars and metal blinds. No AC. No "western" toilet (for guys, no prob). No running water. No sheets for the "cots". You have to sleep with one eye open lest someone steals your stuff. Why did we do it? Beats me...maybe wanted an adventure? We got one. :-) Unless they have upgraded the service -- beware.
The Lahore-Delhi (Samjhota Express) train leaves Lahore at 11:00 hrs on Mondays and Thursdays.(Currently this service is closed)
Check-in time at Lahore Railway Station is 08:00 hrs. First Class fare to Delhi is approx.US$ 10,
Economy Class is US$ 3 whereas Lahore Amritsar First Class is US$ 2 and Economy Class is US$ 1 per son.
From Iran :
Pak-Iran border crossing is only possible through Taftan-Zahidan, from 09:00 hrs to 17:00 hrs with break from 13:00 hrs to 14:00 hrs.
There is a weekly express train from Zahidan to Quetta (via Taftan - 27 hours).
Economy Class fare with berth is approximately around US$ 10 and First Class Sleeper is US$ 13 per person. Another Passenger (slower) train leaves Taftan every Thursday for Quetta.