My friends will be in Baltimore for a wedding, and then they want to travel by train to Philadelphia and to Washington D.C. They looked in internet for the train tickets, but somebody advised them to wait and to buy them there, a day or two before travelling, as in this case the price will be half of the internet price.
Is this true?

The only train service is via Amtrak. I do not believe Amtrak discounts their tickets like that, but I'd suggest checking the Amtrak website.

There are also many trains that require a reserved ticket (not even factoring in the Acela trains) and the trains in the northeast corridor do sell out on busy weekends.

That's not something I've experienced with Amtrak. I have purchased tickets an hour beforehand, and they've been the same price as if I booked them months in advance.

Never heard that before. Many if not most of Amtrak tickets are like airline tickets - more expensive the later you wait. And as Ryan mentioned, it won't help if the train is already sold out.

Thanks to you all for the clarification.

I called the Amtrak 800 number yesterday for another location and found that the time I was looking for was sold out. That is certainly something to consider.

Between Baltimore and DC, they can just ride the MARC trains. Same track, more stops. But only weekdays. $7 each way.

O, Rkkwan, I don't know where my friends found the prices, but the told me about 75 $ / ticket !

Thanks Traveler2005, I'll them about this.

MARC is commuter rail, so very cheap compared to Amtrak along the same route between Baltimore and DC.

For Baltimore to Philadelphia, I believe the only train option is Amtrak. Amtrak prices vary by service (less stops versus more stops, with Acela most expensive) and sometimes by time of day. I've gotten a cheaper ticket on the same type of train by taking the 7AM train instead of the 8AM train, but this might not always be possible. Also, I believe AAA members can get discounts.

MARC only gets to Perryville from Baltimore, and SEPTA only gets to Newark, DE from Philly. So, there is a gap.

It's theoretically possible to take those commuter trains for part of the way between Baltimore and Philly, but probably not worth the trouble.