Porters at the train station wear red uniforms and have badges that say "Porter". They are supposed to charge Rs. 40 per bag, but always want at least Rs. 50 per bag from a tourist or more if you clearly can't manage them yourself. You should fix the price before they touch your bags. I try to travel light and never use porters, though there have been times I wished I had.
New Delhi station is BIG, and if your train happens to be on platform ten, it's a long haul and you might be very happy to pay a porter. They are also good at finding your seat and getting your bags onto the train. They can also be useful once you get off a train if you have a lot of stuff. The second the train stops, lean out the door and wave to one. If there isn't one in sight, (rare) take your bags out as quickly as you can manage and keep them near the train. Try not to leave your bags unattended as this is one of the times things tend to disappear. Once on the train, the Traveling Ticket Examiner (TTE) will come along and check your name off a computerized list. It might be useful to ask him how many stops there are before Mathura Junction. For many express trains, it's the first stop. When the train pulls into the Mathura Junction station, you really have to be standing near the door with your bags prepared to jump off ahead of the crush of people coming on to the train, since they don't generally stop for long. This is more of a problem with the second class unreserved (cattle) cars. There will be many porters, rickshaw, auto rickshaw, and taxi wallahs there to greet you. Which mode you choose depends on where you are going. A taxi will cost about Rs. 300 to Vrindaban. For the economy minded there is also an auto rickshaw which is much less comfortable for Rs. 130 (there is a traffic police pre-paid booth for auto rickshaws). The fare to the Mathura Math is Rs. 30 by auto rickshaw and Rs. 20 by bike rickshaw. You tell them that you need to go Vikash Market, which is near the Math. There may be times when the prepaid booth is unmanned (as it was when I arrived) when haggling is required. The fares stated above should be considered the maximum you should pay in that case and should not present any problem for you. There used to be tempos from the Mathura Junction station, but there aren't anymore as the city of Mathura has banned tempos from certain parts of the city due to noise and air pollution concerns.