I know we can all look this up for ourselves.  But here are two guides
I found online.

From http://www.infoplease.com/ipa/A0001610.html:

| Apostrophe
|  
| Use an apostrophe to indicate:
| 
|   *  The possessive case of singular and plural nouns, indefinate
|      pronouns, and proper nouns:
| 
|        my sister's son
|        my two sisters' sons
|        the children's toys
|        somebody's lunch
|        Charles's house
| 
|   *  The plural of letters, numbers, symbols, and words used as such:
| 
|        too many thus's
|        spelled with two e's
|        ten 5's in a row
|        delete some &'s
| 
|   *  Missing letters in contractions and missing numbers in dates:
| 
|        I'm (I am)
|        ma'am (madam)
|        class of '95
|        winter of '97-'98

From http://www.ous.pdx.edu/FRINQ_Goals/paradigm/punctuat.htm:

|   Indicating Possession.  In the strictest sense the relationship of
|   possession is the relationship of ownership.  In writing, this
|   relationship is indicated by adding an 's to show singular possession
|   and to show plural possession when the standard plural form doesn't
|   end in s.  Possessives of plurals that end in s are formed by adding
|   just the apostrophe.
|
|      Singular possessives:  the boy's hands, the heart's heat, the dog's
|      fleas, Monday's paper, the bass's scales, Charles's snowshoes.
|
|      Plural possessives:  the boys' hands, the dogs' fleas, the basses'
|      scales, the geese's honking, the women's caucus, the mice's holiday.
|
|   Notice that while possession often includes ownership, it sometimes 
|   does not.  For example, while Charles clearly owns his snowshoes, it's
|   less clear that the dog owns its fleas.  In fact, the fleas may think
|   they own the dog.  Still, granting the absence of legal ownership, we
|   can say that the dog "has" fleas.  But what about Monday's paper?  The
|   paper is not possessed by a day of the week.  Yet we consider it to be
|   the "paper of Monday" just as the fleas are "the fleas of the dog."
|   Somehow they go together.  That's what the apostrophe shows us.
|
|   A special case.  One word that doesn't conform to the guidelines above
|   is "its."  Because the word is used so frequently, it's worth learning
|   its ways.  When "its" is a contraction for "it is," it takes an
|   apostrophe.  When "its" indicates possession, no apostrophe is used.

The plural of Russ is Russes.

    Russ's snowshoes      (belonging to Russ)
    The Russes' snowshoes (belonging to the two or more people all named Russ)

The plural of MTA would be MTA's.

-- Chuck