I was wandering in transpontine London...

Here's what the Oxford English Dictionary has to say about the word Transpontine:

1. That is across or over a bridge; spec. on the other side of the bridges in London, i.e. south of the Thames; transf. (from the style of drama in vogue in the 19th century at the ‘Surrey-side’ theatres), melodramatic, sensational.

1844 ALB. SMITH Fort. Scatterg. Fam. ix, It was Monday evening, sacred to the pits and galleries of transpontine theatres. 1860 MRS. W. P. BYRNE Undercurrents Overlooked I. 78 The..Metropolitan theatres, cispontine and transpontine. 1876 C. M. DAVIES Unorth. Lond. 130, I was wandering in transpontine London one Sunday morning. 1882 H. DE WINDT Equator 132 Triana, a transpontine suburb [of Seville], is worth a visit in the daytime. 1901 Scotsman 9 Apr. 5/4 A new drama strongly seasoned with transpontine flavour.

2. [f. L. pontus sea.] That is across the sea; on or from the other side of the ocean, spec. the Atlantic, i.e. North American.

1891 R. L. STEVENSON Let. Oct. (1923) XXII. 414 The last four chapters of The Wrecker!.. Ours is such rude, transpontine business. 1920 Times Lit. Suppl. 15 Apr. 232/2 She [sc. an American writer] has investigated her subject with typical transpontine enthusiasm. 1922 JOYCE Ulysses 416 Thou sawest thy America, thy life~task, and didst charge to cover like the transpontine bison.

I hadn't realized that the word was sometimes used to refer to America - a usage that seems to have faded, not surprizingly as of course there is no bridge across the Atlantic!