I live in a home built in 1910. We put a new furnace/air conditioning unit (Train) in 2005. It seems that the house is a lot colder than it use to be especially the upstairs & the foyer, the room where the cold air register is located. The register is about 3' x 2'. Is it still necessary to have a cold register with such a newer furnace? If so, what is the purpose of the cold air register? Thank you.

The cold air "register" is typically called a return.

The furnace draws air from there and heats it. In reality there should be hot/cold air returns in most rooms with easy to close louvers. The cold air returns are located at the floor and the hot air returns are located near the ceiling. In summer you close the bottom and open the top and in winter you do the opposite.

In reality its called a "Return", the seasons determine if it's returning warm or cold air to be cooled or heated respectively.

Furnaces these days are a lot more efficient and therefore don't heat the space surrounding them unless there is an outlet there. That's probably why it's colder.

Are you keeping the air filter clean? Are all the registers open and not blocked? Have you tries to increase the blower speed?