Trst is beautiful, but is it pronounceable?
Trst (the Croatian and Slovenian version of the name of the beautiful Italian city of Trieste in north-eastern Italy) is a word that looks thoroughly unpronounceable, consisting of nothing but a compact block of consonants. Equally menacing to the English-speaker is the appearance of the word Krk (the name of a nice island in the Croatian archipelago in the northern Adriatic Sea), the name of the Rijeka suburb Trsat or for that matter a whole long list of Slavonic consonants-only words or consonant-dense word elements.
Most English-speakers, Dutch-speakers (and speakers of languages with similar phonetic characteristics) assume that in order to be able to pronounce Trst, Krk and other consonantal Slavonic words, you would need a transitional vowel, a "schwa" (denoted by the phonetic symbol "upside-down e"), between the consonants. Well, there is actually no need for a schwa, as I will try to impress on you in the following.
Sustainably continuant speech
The secret lies in the character of the Croatian (and Slovenian, Serbian, etc.) "R", which is of the rolling variety, like in Spanish, Scottish and several other European languages. The interesting feature of the rolled "R" in this particular connection is that it is continuant or sustainable - you can keep rolling your "RRRRRRRRRRRRRR" for as long as you please, often turning the R into a voiced consonant in the process. Examples of continuant consonants in English are "SSSS", "MMMM", "FFFF" and "LLLL", while the consonants "T", "K" and "P" are non-continuant. Non-continuant consonants can be pronounced in one single burst only. In order to be able to pronounce a second non-continuant consonant in a word, you first need to "re-charge your consonant-pronouncing batteries", by taking a run in a vowel, like you do in the words "tat", "cock", "pop", etc. At the very least, you would need a schwa between two non-continuant consonants in order to make them pronounceable.
Now, what makes continuant consonants interesting is that they can be seamlessly combined with other consonants, i.e. the combination needs no intervening vowel or schwa to be pronounceable. For example, you don't encounter any problems when trying to pronounce the words SSStockholm, SSSSkating, SSSpelling or PHthalate.
Non-sustainable problems of non-continuance
Let's return to our original stumbling-block, the Croatian name of the fair city of Trieste, TRST. As we have just shown, the ending -ST is easily pronounceable for all, as it consists of a continuant, sustainable S, followed by the non-sustainable (non-continuant) T. What about pronouncing the beginning TR-, when it is followed by a consonant ? For an Englishman or Dutchwoman this would be impossible, of course, because the English and Dutch R's are not only non-continuant, but also very far from a rolled R in other phonetic respects. Furthermore, the T in English is pronounced "thickly", with the tongue situated far from where it needs to be to start pronouncing a subsequent English non-rolled R.
However, for people proficient in pronouncing rolling continuant R's and tip-of-the-tongue T's, there is no corresponding problem. The non-continuant T goes seamlessly over to the continuant R, and then - equally seamlessly - to the continuant S, followed by the final non-continuant T. Hence Trst is easily pronounceable for people who have mastered the rolling R, without any need for a schwa or two.
Unlike, like apples and bananas
So, to finish this long-winded story with a succinct summary - the apparent physical impossibility of pronouncing Trst without any intervening schwa's stems from the fact that the R in the Croatian word denotes a completely different phonetic entity than R does in English words.
Thanks to Gritchka for the correct technical term continuant and many clarifying observations regarding comparative phonetics !