<PAGE>   1

                                                                     EXHIBIT 4.2

               FORM OF SERIES B EXCHANGE PASS THROUGH CERTIFICATE



                 Reliant Energy Mid-Atlantic Power Holdings, LLC
                           SERIES B PASS THROUGH TRUST

                9.237% Series B Exchange Pass Through Certificate


                                     CUSIP:

                      Final Distribution Date: July 2, 2017

                  evidencing a fractional undivided interest in a trust, the
                  property of which includes certain notes secured by certain
                  property leased to Reliant Energy Mid-Atlantic Power Holdings,
                  LLC.

Certificate No.                          $_______ Fractional Undivided Interest

                  THIS CERTIFIES THAT ____________________ for value received,
is the registered owner of a $___________ (___________________) ( _________
dollars) Fractional Undivided Interest in the Reliant Energy Mid-Atlantic Power
Holdings, LLC-Series B Pass Through Trust (the "Pass Through Trust") created
pursuant to a Pass Through Trust Agreement dated as of August 24, 2000 (the
"Agreement") between Bankers Trust Company, as trustee (the "Pass Through
Trustee") and Reliant Energy Mid-Atlantic Power Holdings, LLC, a limited
liability company incorporated under Delaware law (the "Company"), a summary of
certain of the pertinent provisions of which is set forth below. To the extent
not otherwise defined herein, the capitalized terms used herein have the
meanings assigned to them in the Agreement. This Certificate is one of the duly
authorized Certificates designated as "9.237% Series B Exchange Pass Through
Certificates" (herein called the "Certificates"). This Certificate is issued
under and is subject to the terms, provisions and conditions of the Agreement,
to which Agreement the Holder of this Certificate by virtue of the acceptance
hereof assents and by which such Holder is bound. The property of the Pass
Through Trust includes certain Lessor Notes (the "Trust Property"). The Lessor
Notes are secured by a security interest in the Facility, and liability
thereunder is limited to the income and proceeds of such security.

                  Subject to and in accordance with the terms of the Agreement,
from funds then available to the Pass Through Trustee, there will be distributed
on each January 2 and July 2 (a "Distribution Date"), commencing on July 2,
2001, to the person in whose name this Certificate is registered at the close of
business on the day of the month which is 15 days preceding the Distribution
Date, an amount in respect of the Scheduled Payments on the Lessor Notes due on
such Distribution Date, the receipt of which has been confirmed by the Pass
Through Trustee, equal to the product of the percentage interest in the Pass
Through Trust evidenced by this Certificate and an amount equal to the sum of
such Scheduled Payments. Subject to and in accordance with the terms of the
Agreement, in the event that Special Payments on the Lessor Notes are received
by the Pass Through Trustee, from funds then available to the Pass Through



<PAGE>   2


Trustee, there shall be distributed on the applicable Special Distribution Date,
to the Person in whose name this certificate is registered at the close of
business on the day of the month which is 15 days preceding the Special
Distribution Date, an amount in respect of such Special Payments on the Lessor
Notes, the receipt of which has been confirmed by the Pass Through Trustee,
equal to the product of the percentage interest in the Pass Through Trust
evidenced by this Certificate and an amount equal to the sum of such Special
Payments so received. The Special Distribution Date shall be determined as
provided in the Agreement. If a Distribution Date or Special Distribution Date
is not a Business Day, distribution shall be made on the immediately following
Business Day. The Pass Through Trustee shall mail notice of each Special Payment
and the Special Distribution Date therefor to the Holders of the Certificates.

                  Distributions on this Certificate will be made by the Pass
Through Trustee by (i) if (A) The Depository Trust Company, a New York
corporation ("DTC") is the Certificateholder of record of this Certificate, or
(B) a Certificateholder holds a Certificate or Certificates in an aggregate
amount greater than $10,000,000, or (C) a Certificateholder holds a Certificate
or Certificates in an aggregate amount greater than $1,000,000 and so requests
to the Pass Through Trustee, by wire transfer in immediately available funds to
an account maintained by such Certificateholder with a bank, or (ii) if none of
the above apply, by check mailed to such Certificateholder at the address
appearing in the Register, without the presentation or surrender of this
Certificate or the making of any notation hereon. Except as otherwise provided
in the Agreement and notwithstanding the above, the final distribution on this
Certificate will be made after notice mailed by the Pass Through Trustee of the
pendency of such distribution and only upon presentation and surrender of this
Certificate at the office or agency of the Pass Through Trustee specified in
such notice.

                  Unless this certificate is presented by an authorized
representative of DTC to the Company or its agent for registration of transfer,
exchange or payment, and any certificate issued is registered in the name of
Cede & Co. or in such other name as is requested by an authorized representative
of DTC (and any payment is made to Cede & Co. or to such other entity as is
requested by an authorized representative of DTC), ANY TRANSFER, PLEDGE OR OTHER
USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL in as much as
the registered owner hereof, Cede & Co., has an interest herein.

                  Each Person who acquires or accepts this Certificate or an
interest herein will be deemed by such acquisition or acceptance to have
represented and warranted that either: (i) no Plan assets have been used to
purchase this Certificate or an interest herein or (ii) the purchase and holding
of this Certificate or interest herein are exempt from the prohibited
transaction restrictions of ERISA and the Code pursuant to one or more
prohibited transaction statutory or administrative exemptions.

                  This Certificate shall be governed by and construed in
accordance with the laws of the State of New York.

                  Reference is hereby made to the further provisions of this
Certificate set forth on the reverse hereof, which further provisions shall for
all purposes have the same effect as if set forth at this place.


<PAGE>   3


                  Unless the certificate of authentication hereon has been
executed by the Pass Through Trustee, by manual signature, this Certificate
shall not be entitled to any benefit under the Agreement or be valid for any
purpose.

                  IN WITNESS WHEREOF, the Pass Through Trustee has caused this
Certificate to be duly executed.

                                         RELIANT ENERGY MID-ATLANTIC
                                         POWER HOLDINGS, LLC-
                                         SERIES B PASS THROUGH TRUST


                                         By: Bankers Trust Company, as Pass
                                             Through Trustee


                                               By:
                                                  --------------------------
                                           Title:


<PAGE>   4


                            [Reverse Of Certificate]

                  The Certificates do not represent a direct obligation of, or
an obligation guaranteed by, or an interest in, the Company, the Owner Lessor,
the Owner Participant, the Equity Investor, the Equity Subsidiary, the Equity
Subsidiary Holding Company, the Pass Through Trust Company or the Pass Through
Trustee or any affiliate thereof. The Certificates are limited in right of
payment, all as more specifically set forth in the Agreement. All payments or
distributions made to Certificateholders under the Agreement shall be made only
from the Trust Property and only to the extent that the Pass Through Trustee
shall have received sufficient income or proceeds from the Trust Property to
make such payments in accordance with the terms of the Agreement. Each Holder of
this Certificate, by its acceptance hereof, agrees that it will look solely to
the income and proceeds from the Trust Property to the extent available for
distribution to such Holder as provided in the Agreement. This Certificate does
nor purport to summarize the Agreement and reference is made to the Agreement
for information with respect to the interests, rights, benefits, obligations,
proceeds and duties evidenced hereby. A copy of the Agreement may be examined
during normal business hours at the principal office of the Pass Through
Trustee, and at such other places, if any, designated by the Pass Through
Trustee, by any Certificateholder upon request.

                  The Agreement permits, with certain exceptions therein
provided, the amendment thereof and the modification of the rights and
obligations of the Company and the rights of the Certificateholders under the
Agreement at any time by the Company and the Pass Through Trustee with the
consent of the Holders of Certificates evidencing Fractional Undivided Interests
aggregating not less than a majority in interest of the Fractional Undivided
Interests evidenced by all Certificates at the time Outstanding. Any such
consent by the Holder of this Certificate shall be conclusive and binding on
such Holder and upon all future Holders of this Certificate and of any
Certificate issued upon the transfer hereof or in exchange hereof or in lieu
hereof whether or not notation of such consent is made upon this Certificate.
The Agreement also permits the amendment thereof, in certain limited
circumstances, without the consent of the Holders of any of the Certificates.

                  As provided in the Agreement and subject to certain
limitations therein set forth, the transfer of this Certificate is registrable
in the Register upon surrender of this Certificate for registration of transfer
at the offices or agencies maintained by the Pass Through Trustee in its
capacity as Registrar, or by any successor Registrar, in the Borough Of
Manhattan, the City of New York, duly endorsed or accompanied by a written
instrument of transfer in form satisfactory to the Pass Through Trustee and the
Registrar duly executed by the Holder hereof or such Holder's attorney duly
authorized in writing, and thereupon one or more new Certificates of authorized
denominations evidencing the same aggregate Fractional Undivided Interest in the
Pass Through Trust will be issued to the designated transferee or transferees.

                  The Certificates are issuable only as registered Certificates
without coupons in minimum denominations of $100,000 Fractional Undivided
Interest and any integral multiples of $1,000 in excess thereof. As provided in
the Agreement and subject to certain limitations therein set forth, Certificates
are exchangeable for new Certificates of authorized denominations evidencing the
same aggregate Fractional Undivided Interest in the Pass Through Trust, as
requested by the Holder surrendering the same.


<PAGE>   5


                  No service charge will be made for any such registration of
transfer or exchange, but the Pass Through Trustee shall require payment of a
sum sufficient to cover any tax or governmental charge payable in connection
therewith.

                  The Pass Through Trustee, the Company, the Owner Lessor, the
Registrar and any agent of the Pass Through Trustee or the Registrar may treat
the person in whose name this Certificate is registered as the owner hereof for
all purposes, and neither the Pass Through Trustee, the Company, the Owner
Lessor, the Registrar nor any such agent shall be affected by any notice to the
contrary.

                  The obligations and responsibilities created by the Agreement
and the Pass Through Trust created thereby shall terminate upon the distribution
to Certificateholders of all amounts required to be distributed to them pursuant
to the Agreement and the disposition of all property held as part of the Trust
Property.