Consuming your own food in many of the lounges is not permitted. It varies by route or location in a car, like an ssl, ok upstairs, not allowed downstairs where food is sold Amtrak website says:
You may bring your own food and beverages onboard for consumption at your seat or private Sleeping Car accommodations. However, you can only consume food and beverages purchased in Dining and Lounge Cars in those cars. Personal food and beverages are allowed in the upper level of Superliner Sightseer Lounges.
Service Manual is slightly different since it points out the primary exceptions
11. Non-Amtrak Food and Beverage (Non Alcohol)
a) May be consumed by passengers in the following
• Any Coach seat.
• Any Sleeping Car accommodation (both alcohol
and non-alcohol beverages permitted).
• Upper level of Superliner Lounge Cars.
• Acela Express equipment.
• Empire Service equipment.
• Northeast Regional Café Cars.
b) All other Food Service Cars are intended for the
consumption of only Amtrak provided meals,
snacks and beverages.